diff --git a/CHANGELOG.md b/CHANGELOG.md
index 144292117fa..d0da24a7afc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## **[Unreleased]**
 
+- [#936](https://github.com/wasmerio/wasmer/pull/939) Fix bug causing attempts to append to files with WASI to delete the contents of the file
 - [#940](https://github.com/wasmerio/wasmer/pull/940) Update supported Rust version to 1.38+
 - [#921](https://github.com/wasmerio/wasmer/pull/921) In LLVM backend, annotate all memory accesses with TBAA metadata.
 - [#883](https://github.com/wasmerio/wasmer/pull/883) Allow floating point operations to have arbitrary inputs, even including SNaNs.
diff --git a/lib/wasi-tests/build/wasitests.rs b/lib/wasi-tests/build/wasitests.rs
index 0144da92056..5ccda63c9fe 100644
--- a/lib/wasi-tests/build/wasitests.rs
+++ b/lib/wasi-tests/build/wasitests.rs
@@ -83,7 +83,7 @@ pub fn compile(file: &str, ignores: &HashSet<String>) -> Option<String> {
         .arg("+nightly")
         .arg("--target=wasm32-wasi")
         .arg("-C")
-        .arg("opt-level=s")
+        .arg("opt-level=z")
         .arg(file)
         .arg("-o")
         .arg(&wasm_out_name)
diff --git a/lib/wasi-tests/tests/wasitests/fd_append.rs b/lib/wasi-tests/tests/wasitests/fd_append.rs
new file mode 100644
index 00000000000..0ac3ea6ebf8
--- /dev/null
+++ b/lib/wasi-tests/tests/wasitests/fd_append.rs
@@ -0,0 +1,18 @@
+// !!! THIS IS A GENERATED FILE !!!
+// ANY MANUAL EDITS MAY BE OVERWRITTEN AT ANY TIME
+// Files autogenerated with cargo build (build/wasitests.rs).
+
+#[test]
+fn test_fd_append() {
+    assert_wasi_output!(
+        "../../wasitests/fd_append.wasm",
+        "fd_append",
+        vec![],
+        vec![(
+            ".".to_string(),
+            ::std::path::PathBuf::from("wasitests/test_fs/temp")
+        ),],
+        vec![],
+        "../../wasitests/fd_append.out"
+    );
+}
diff --git a/lib/wasi-tests/tests/wasitests/mod.rs b/lib/wasi-tests/tests/wasitests/mod.rs
index 75a91d6f003..d226dc0ee0d 100644
--- a/lib/wasi-tests/tests/wasitests/mod.rs
+++ b/lib/wasi-tests/tests/wasitests/mod.rs
@@ -9,6 +9,7 @@ mod close_preopen_fd;
 mod create_dir;
 mod envvar;
 mod fd_allocate;
+mod fd_append;
 mod fd_close;
 mod fd_pread;
 mod fd_read;
diff --git a/lib/wasi-tests/wasitests/close_preopen_fd.wasm b/lib/wasi-tests/wasitests/close_preopen_fd.wasm
index 56d37f64630..5badfb755e5 100755
Binary files a/lib/wasi-tests/wasitests/close_preopen_fd.wasm and b/lib/wasi-tests/wasitests/close_preopen_fd.wasm differ
diff --git a/lib/wasi-tests/wasitests/create_dir.wasm b/lib/wasi-tests/wasitests/create_dir.wasm
index f4a9898f0c7..0c2fb0bbf08 100755
Binary files a/lib/wasi-tests/wasitests/create_dir.wasm and b/lib/wasi-tests/wasitests/create_dir.wasm differ
diff --git a/lib/wasi-tests/wasitests/envvar.wasm b/lib/wasi-tests/wasitests/envvar.wasm
index 1e394c49f97..b05151905a2 100755
Binary files a/lib/wasi-tests/wasitests/envvar.wasm and b/lib/wasi-tests/wasitests/envvar.wasm differ
diff --git a/lib/wasi-tests/wasitests/fd_allocate.wasm b/lib/wasi-tests/wasitests/fd_allocate.wasm
index ee688bf93ad..e604f6e407d 100755
Binary files a/lib/wasi-tests/wasitests/fd_allocate.wasm and b/lib/wasi-tests/wasitests/fd_allocate.wasm differ
diff --git a/lib/wasi-tests/wasitests/fd_append.out b/lib/wasi-tests/wasitests/fd_append.out
new file mode 100644
index 00000000000..e9d92ef26a6
--- /dev/null
+++ b/lib/wasi-tests/wasitests/fd_append.out
@@ -0,0 +1 @@
+"Hello, world!\nGoodbye, world!\n"
diff --git a/lib/wasi-tests/wasitests/fd_append.rs b/lib/wasi-tests/wasitests/fd_append.rs
new file mode 100644
index 00000000000..f1e55c5ed9a
--- /dev/null
+++ b/lib/wasi-tests/wasitests/fd_append.rs
@@ -0,0 +1,48 @@
+// Args:
+// mapdir: .:wasitests/test_fs/temp
+
+use std::fs::OpenOptions;
+use std::io::{Read, Write};
+use std::path::PathBuf;
+
+static STR1: &str = "Hello, world!\n";
+static STR2: &str = "Goodbye, world!\n";
+
+fn main() {
+    let file = {
+        #[cfg(not(target_os = "wasi"))]
+        let mut base = PathBuf::from("wasitests/test_fs/temp");
+        #[cfg(target_os = "wasi")]
+        let mut base = PathBuf::from(".");
+
+        base.push("fd_append_test");
+        base
+    };
+
+    {
+        let mut file_handle = OpenOptions::new()
+            .create_new(true)
+            .append(true)
+            .open(&file)
+            .expect("Couldn't create file");
+        file_handle.write(STR1.as_bytes()).unwrap();
+    }
+    {
+        let mut file_handle = OpenOptions::new()
+            .append(true)
+            .open(&file)
+            .expect("Couldn't reopen file to append");
+        file_handle.write(STR2.as_bytes()).unwrap();
+    }
+
+    let mut file_handle = OpenOptions::new()
+        .read(true)
+        .open(&file)
+        .expect("Couldn't reopen file to read");
+
+    let mut test = String::new();
+    file_handle.read_to_string(&mut test);
+
+    assert_eq!(&test, &format!("{}{}", STR1, STR2));
+    println!("{:?}", &test);
+}
diff --git a/lib/wasi-tests/wasitests/fd_append.wasm b/lib/wasi-tests/wasitests/fd_append.wasm
new file mode 100755
index 00000000000..b1c73d14fab
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diff --git a/lib/wasi-tests/wasitests/fd_close.wasm b/lib/wasi-tests/wasitests/fd_close.wasm
index 5ace5206203..162e968755a 100755
Binary files a/lib/wasi-tests/wasitests/fd_close.wasm and b/lib/wasi-tests/wasitests/fd_close.wasm differ
diff --git a/lib/wasi-tests/wasitests/fd_pread.wasm b/lib/wasi-tests/wasitests/fd_pread.wasm
index 11031b32ef0..7b48e4959fc 100755
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diff --git a/lib/wasi-tests/wasitests/fd_read.wasm b/lib/wasi-tests/wasitests/fd_read.wasm
index 227157f989d..799ca164514 100755
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diff --git a/lib/wasi-tests/wasitests/fd_sync.wasm b/lib/wasi-tests/wasitests/fd_sync.wasm
index 497b63c9bc8..5fde9b62744 100755
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diff --git a/lib/wasi-tests/wasitests/file_metadata.wasm b/lib/wasi-tests/wasitests/file_metadata.wasm
index 938fdc1f256..68c59917238 100755
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diff --git a/lib/wasi-tests/wasitests/fs_sandbox_test.wasm b/lib/wasi-tests/wasitests/fs_sandbox_test.wasm
index 3f088985069..8a8c6349451 100755
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diff --git a/lib/wasi-tests/wasitests/fseek.wasm b/lib/wasi-tests/wasitests/fseek.wasm
index ce68b75f05a..d3a7c8f8494 100755
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diff --git a/lib/wasi-tests/wasitests/hello.wasm b/lib/wasi-tests/wasitests/hello.wasm
index 1175a7f7236..a2625868b71 100755
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diff --git a/lib/wasi-tests/wasitests/mapdir.wasm b/lib/wasi-tests/wasitests/mapdir.wasm
index 849a3a6ae2f..5675534ef0b 100755
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diff --git a/lib/wasi-tests/wasitests/path_link.wasm b/lib/wasi-tests/wasitests/path_link.wasm
index 306a911e651..eae511069d0 100755
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diff --git a/lib/wasi-tests/wasitests/path_rename.wasm b/lib/wasi-tests/wasitests/path_rename.wasm
index f76120ab85a..7e613b96011 100755
Binary files a/lib/wasi-tests/wasitests/path_rename.wasm and b/lib/wasi-tests/wasitests/path_rename.wasm differ
diff --git a/lib/wasi-tests/wasitests/path_symlink.wasm b/lib/wasi-tests/wasitests/path_symlink.wasm
index 61bb0a0224b..c30b1637864 100755
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diff --git a/lib/wasi-tests/wasitests/poll_oneoff.wasm b/lib/wasi-tests/wasitests/poll_oneoff.wasm
index 9510f5b83ee..009a57e8061 100755
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diff --git a/lib/wasi-tests/wasitests/quine.wasm b/lib/wasi-tests/wasitests/quine.wasm
index 8cf55c4aca2..21fb813a378 100755
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diff --git a/lib/wasi-tests/wasitests/readlink.wasm b/lib/wasi-tests/wasitests/readlink.wasm
index 26480cf84aa..d8ef0c74fee 100755
Binary files a/lib/wasi-tests/wasitests/readlink.wasm and b/lib/wasi-tests/wasitests/readlink.wasm differ
diff --git a/lib/wasi-tests/wasitests/wasi_sees_virtual_root.wasm b/lib/wasi-tests/wasitests/wasi_sees_virtual_root.wasm
index 7f6d3125374..0abf7729494 100755
Binary files a/lib/wasi-tests/wasitests/wasi_sees_virtual_root.wasm and b/lib/wasi-tests/wasitests/wasi_sees_virtual_root.wasm differ
diff --git a/lib/wasi-tests/wasitests/writing.wasm b/lib/wasi-tests/wasitests/writing.wasm
index ce7e0180a7d..0b8b96abcdf 100755
Binary files a/lib/wasi-tests/wasitests/writing.wasm and b/lib/wasi-tests/wasitests/writing.wasm differ
diff --git a/lib/wasi/src/syscalls/mod.rs b/lib/wasi/src/syscalls/mod.rs
index 5854690352d..3db7323e7c9 100644
--- a/lib/wasi/src/syscalls/mod.rs
+++ b/lib/wasi/src/syscalls/mod.rs
@@ -1724,12 +1724,25 @@ pub fn path_open(
                     }
                 }
                 let mut open_options = std::fs::OpenOptions::new();
+                let write_permission = adjusted_rights & __WASI_RIGHT_FD_WRITE != 0;
+                // append, truncate, and create all require the permission to write
+                let (append_permission, truncate_permission, create_permission) =
+                    if write_permission {
+                        (
+                            fs_flags & __WASI_FDFLAG_APPEND != 0,
+                            o_flags & __WASI_O_TRUNC != 0,
+                            o_flags & __WASI_O_CREAT != 0,
+                        )
+                    } else {
+                        (false, false, false)
+                    };
                 let open_options = open_options
                     .read(true)
                     // TODO: ensure these rights are actually valid given parent, etc.
-                    .write(adjusted_rights & __WASI_RIGHT_FD_WRITE != 0)
-                    .create(o_flags & __WASI_O_CREAT != 0)
-                    .truncate(o_flags & __WASI_O_TRUNC != 0);
+                    .write(write_permission)
+                    .create(create_permission)
+                    .append(append_permission)
+                    .truncate(truncate_permission);
                 open_flags |= Fd::READ;
                 if adjusted_rights & __WASI_RIGHT_FD_WRITE != 0 {
                     open_flags |= Fd::WRITE;
@@ -1798,6 +1811,7 @@ pub fn path_open(
                 let mut open_options = std::fs::OpenOptions::new();
                 let open_options = open_options
                     .read(true)
+                    .append(fs_flags & __WASI_FDFLAG_APPEND != 0)
                     // TODO: ensure these rights are actually valid given parent, etc.
                     // write access is required for creating a file
                     .write(true)