diff --git a/fixtures/merged-root-fields/a.graphql b/fixtures/merged-root-fields/a.graphql
index b2d361f..6ec2ae3 100644
--- a/fixtures/merged-root-fields/a.graphql
+++ b/fixtures/merged-root-fields/a.graphql
@@ -1,4 +1,4 @@
-# import Query.* from 'b.graphql'
+# import Query.*, Dummy.* from 'b.graphql'
 
 type Query {
     helloA: String
diff --git a/fixtures/merged-root-fields/b.graphql b/fixtures/merged-root-fields/b.graphql
index f1d3db5..c580b95 100644
--- a/fixtures/merged-root-fields/b.graphql
+++ b/fixtures/merged-root-fields/b.graphql
@@ -9,4 +9,8 @@ type Post {
 
 input PostFilter {
     field3: Int
+}
+
+type Dummy {
+    field2: String
 }
\ No newline at end of file
diff --git a/src/definition.ts b/src/definition.ts
index e9f2b2a..1766cd9 100644
--- a/src/definition.ts
+++ b/src/definition.ts
@@ -1,9 +1,6 @@
 import { keyBy, uniqBy, includes } from 'lodash'
 import {
-  DocumentNode,
   TypeDefinitionNode,
-  ObjectTypeDefinitionNode,
-  InputObjectTypeDefinitionNode,
   TypeNode,
   NamedTypeNode,
   DirectiveNode,
diff --git a/src/index.test.ts b/src/index.test.ts
index db867f3..a86fa22 100644
--- a/src/index.test.ts
+++ b/src/index.test.ts
@@ -261,7 +261,7 @@ type C2 {
   id: ID!
 }
 `
-  t.is(importSchema(schemaA, schemas), expectedSDL)
+  t.is(importSchema(schemaA, { schemas }), expectedSDL)
 })
 
 test(`importSchema: single object schema`, t => {
@@ -324,7 +324,7 @@ type C2 {
   id: ID!
 }
 `
-  t.is(importSchema(schemaA, schemas), expectedSDL)
+  t.is(importSchema(schemaA, { schemas }), expectedSDL)
 })
 
 test(`importSchema: import all mix 'n match 2`, t => {
@@ -443,7 +443,7 @@ type C2 {
   id: ID!
 }
 `
-  t.is(importSchema(schemaA, schemas), expectedSDL)
+  t.is(importSchema(schemaA, { schemas }), expectedSDL)
 })
 
 test('importSchema: scalar', t => {
@@ -704,6 +704,31 @@ input PostFilter {
   t.is(actualSDL, expectedSDL)
 })
 
+test('merged custom root fields imports', t => {
+  const expectedSDL = `\
+type Query {
+  helloA: String
+  posts(filter: PostFilter): [Post]
+  hello: String
+}
+
+type Dummy {
+  field: String
+  field2: String
+}
+
+type Post {
+  field1: String
+}
+
+input PostFilter {
+  field3: Int
+}
+`
+  const actualSDL = importSchema('fixtures/merged-root-fields/a.graphql', { mergeableTypes: ['Dummy'] })
+  t.is(actualSDL, expectedSDL)
+})
+
 test('global schema modules', t => {
   const shared = `
     type Shared {
@@ -720,7 +745,7 @@ type Shared {
   first: String
 }
 `
-  t.is(importSchema('fixtures/global/a.graphql', { shared }), expectedSDL)
+  t.is(importSchema('fixtures/global/a.graphql', { schemas: { shared } }), expectedSDL)
 })
 
 test('missing type on type', t => {
diff --git a/src/index.ts b/src/index.ts
index 0ab3252..1bfedc6 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -3,8 +3,6 @@ import {
   DefinitionNode,
   parse,
   print,
-  TypeDefinitionNode,
-  GraphQLObjectType,
   ObjectTypeDefinitionNode,
   DocumentNode,
   Kind,
@@ -24,6 +22,14 @@ export interface RawModule {
   from: string
 }
 
+/**
+ * Configuration options that may be passed to `importSchema`
+ */
+interface ImportSchemaOptions {
+  schemas?: { [key: string]: string }
+  mergeableTypes?: [string]
+}
+
 const rootFields = ['Query', 'Mutation', 'Subscription']
 
 const read = (schema: string, schemas?: { [key: string]: string }) => {
@@ -33,7 +39,7 @@ const read = (schema: string, schemas?: { [key: string]: string }) => {
   return schemas ? schemas[schema] : schema
 }
 
-const isFile = f => f.endsWith('.graphql')
+const isFile = (f: string) => f.endsWith('.graphql')
 
 /**
  * Parse a single import line and extract imported types and schema filename
@@ -77,13 +83,20 @@ export function parseSDL(sdl: string): RawModule[] {
 /**
  * Main entry point. Recursively process all import statement in a schema
  *
- * @param filePath File path to the initial schema file
+ * @see https://oss.prisma.io/content/graphql-import/overview#description
+ * @param schema File path to the initial schema file
+ * @param options Import configuration options
+ * @param options.schemas An object of schemas as strings
+ * @param options.mergeableTypes An array of custom GraphQL types that will
+ *  be treated as [root fields]{@link https://oss.prisma.io/content/graphql-import/overview#import-root-fields}
  * @returns Single bundled schema with all imported types
  */
 export function importSchema(
   schema: string,
-  schemas?: { [key: string]: string },
+  options: ImportSchemaOptions = {},
 ): string {
+  const { schemas, mergeableTypes = [] } = options
+  const allMergeableTypes = [...mergeableTypes, ...rootFields]
   const sdl = read(schema, schemas) || schema
   let document = getDocumentFromSDL(sdl)
 
@@ -96,14 +109,15 @@ export function importSchema(
   )
 
   // Post processing of the final schema (missing types, unused types, etc.)
-  // Query, Mutation and Subscription should be merged
+  // Query, Mutation, Subscription and any custom type defined in `mergeableTypes`
+  // should be merged
   // And should always be in the first set, to make sure they
   // are not filtered out.
   const firstTypes = flatten(typeDefinitions).filter(d =>
-    includes(rootFields, d.name.value),
+    includes(allMergeableTypes, d.name.value),
   )
   const otherFirstTypes = typeDefinitions[0].filter(
-    d => !includes(rootFields, d.name.value),
+    d => !includes(allMergeableTypes, d.name.value),
   )
   const firstSet = firstTypes.concat(otherFirstTypes)
   const processedTypeNames = []