diff --git a/lib/nmatrix/io/mat5_reader.rb b/lib/nmatrix/io/mat5_reader.rb
index c35fa5f5..589f0184 100644
--- a/lib/nmatrix/io/mat5_reader.rb
+++ b/lib/nmatrix/io/mat5_reader.rb
@@ -47,7 +47,7 @@ def initialize(stream = nil, byte_order = nil, content_or_bytes = nil)
         if content_or_bytes.is_a?(String)
           @content = content_or_bytes
 
-        elsif content_or_bytes.is_a?(Fixnum)
+        elsif content_or_bytes.is_a?(Integer)
           @padded_bytes = content_or_bytes
         end
       end
diff --git a/lib/nmatrix/monkeys.rb b/lib/nmatrix/monkeys.rb
index 7646ba5b..ceaa516c 100644
--- a/lib/nmatrix/monkeys.rb
+++ b/lib/nmatrix/monkeys.rb
@@ -36,7 +36,7 @@ class Array
   # You must provide a shape for the matrix as the first argument.
   #
   # == Arguments:
-  # <tt>shape</tt> :: Array describing matrix dimensions (or Fixnum for square).
+  # <tt>shape</tt> :: Array describing matrix dimensions (or Integer for square).
   #   If not provided, will be intuited through #shape.
   # <tt>dtype</tt> :: Override data type (e.g., to store a Float as :float32
   #   instead of :float64) -- optional.
@@ -46,7 +46,7 @@ def to_nm(shape = nil, dtype = nil, stype = :dense)
 
     guess_dtype = ->(type) {
       case type
-      when Fixnum   then :int64
+      when Integer  then :int64
       when Float    then :float64
       when Complex  then :complex128
       end
diff --git a/lib/nmatrix/nmatrix.rb b/lib/nmatrix/nmatrix.rb
index 60793380..9fb8d1cf 100644
--- a/lib/nmatrix/nmatrix.rb
+++ b/lib/nmatrix/nmatrix.rb
@@ -235,7 +235,7 @@ def pretty_print(q) #:nodoc:
   #
   # If your dtype is :object and you are converting from :dense to a sparse type, it is recommended that you
   # provide a :default, as 0 may behave differently from its Float or Complex equivalent. If no option
-  # is given, Fixnum 0 will be used.
+  # is given, Integer 0 will be used.
   def cast(*params)
     if (params.size > 0 && params[0].is_a?(Hash))
       opts = {
@@ -419,7 +419,7 @@ def to_flat_array
 
   #
   # call-seq:
-  #     size -> Fixnum
+  #     size -> Integer
   #
   # Returns the total size of the NMatrix based on its shape.
   #
@@ -566,12 +566,12 @@ def last
   # the new and old shapes' components must be equal.
   #
   # * *Arguments* :
-  #   - +new_shape+ -> Array of positive Fixnums.
+  #   - +new_shape+ -> Array of positive Integers.
   # * *Returns* :
   #   - A copy with a different shape.
   #
   def reshape new_shape,*shapes
-    if new_shape.is_a?Fixnum
+    if new_shape.is_a?Integer
       newer_shape =  [new_shape]+shapes
     else  # new_shape is an Array
       newer_shape = new_shape
@@ -593,13 +593,13 @@ def reshape new_shape,*shapes
   # the new and old shapes' components must be equal.
   #
   # * *Arguments* :
-  #   - +new_shape+ -> Array of positive Fixnums.
+  #   - +new_shape+ -> Array of positive Integer.
   #
   def reshape! new_shape,*shapes
     if self.is_ref?
       raise(ArgumentError, "This operation cannot be performed on reference slices")
     else
-      if new_shape.is_a?Fixnum
+      if new_shape.is_a?Integer
         shape =  [new_shape]+shapes
       else  # new_shape is an Array
         shape = new_shape
@@ -690,7 +690,7 @@ def transpose(permute = nil)
   #
   # * *Arguments* :
   #   - +matrices+ -> one or more matrices
-  #   - +rank+ -> Fixnum (for rank); alternatively, may use :row, :column, or
+  #   - +rank+ -> Integer (for rank); alternatively, may use :row, :column, or
   #   :layer for 0, 1, 2, respectively
   def concat(*matrices)
     rank = nil