diff --git a/src/index.d.ts b/src/index.d.ts
index 2ee8dc6..4711efb 100644
--- a/src/index.d.ts
+++ b/src/index.d.ts
@@ -10,13 +10,13 @@
 /////////////////////////////////////////////////////////////////////////////////////////////
 
 
-export * from './types/crc-calculator';
-export * from './types/decoder';
-export * from './types/encoder';
-export * from './types/mesg';
-export * from './types/mesgs';
-export * from './types/profile';
-export * from './types/stream';
-export * from './types/types';
-export { default as Types } from './types/types';
-export * from './types/utils';
+export * from './types/crc-calculator.js';
+export * from './types/decoder.js';
+export * from './types/encoder.js';
+export * from './types/mesg.js';
+export * from './types/mesgs.js';
+export * from './types/profile.js';
+export * from './types/stream.js';
+export * from './types/types.js';
+export { default as Types } from './types/types.js';
+export * from './types/utils.js';
diff --git a/src/types/decoder.d.ts b/src/types/decoder.d.ts
index a058a5a..5624eeb 100644
--- a/src/types/decoder.d.ts
+++ b/src/types/decoder.d.ts
@@ -10,9 +10,9 @@
 /////////////////////////////////////////////////////////////////////////////////////////////
 
 
-import { Stream } from "./stream";
-import { Mesg } from "./mesg";
-import { FitMessages, DeveloperDataIdMesg, FieldDescriptionMesg } from "./mesgs";
+import { Stream } from "./stream.js";
+import { Mesg } from "./mesg.js";
+import { FitMessages, DeveloperDataIdMesg, FieldDescriptionMesg } from "./mesgs.js";
 
 /** Decodes FIT binary data from a {@link Stream}. */
 export class Decoder {
diff --git a/src/types/encoder.d.ts b/src/types/encoder.d.ts
index c6faa4f..468760b 100644
--- a/src/types/encoder.d.ts
+++ b/src/types/encoder.d.ts
@@ -10,8 +10,8 @@
 /////////////////////////////////////////////////////////////////////////////////////////////
 
 
-import { Encodable, Mesg } from "./mesg";
-import { DeveloperDataIdMesg, FieldDescriptionMesg } from "./mesgs";
+import { Encodable, Mesg } from "./mesg.js";
+import { DeveloperDataIdMesg, FieldDescriptionMesg } from "./mesgs.js";
 
 export interface FieldDescription {
     /** The Developer Data Id message. */
diff --git a/src/types/mesg.d.ts b/src/types/mesg.d.ts
index dfce270..3896a5e 100644
--- a/src/types/mesg.d.ts
+++ b/src/types/mesg.d.ts
@@ -10,7 +10,7 @@
 /////////////////////////////////////////////////////////////////////////////////////////////
 
 
-import Types from "./types";
+import Types from "./types.js";
 
 /** A FIT field value — a scalar or array of one of the primitive FIT base types. */
 export type FieldValue =
diff --git a/src/types/mesgs.d.ts b/src/types/mesgs.d.ts
index 1bd5c86..0924d00 100644
--- a/src/types/mesgs.d.ts
+++ b/src/types/mesgs.d.ts
@@ -10,8 +10,8 @@
 /////////////////////////////////////////////////////////////////////////////////////////////
 
 
-import { Mesg } from "./mesg";
-import Types from "./types";
+import { Mesg } from "./mesg.js";
+import Types from "./types.js";
 
 
 /**
diff --git a/src/types/stream.d.ts b/src/types/stream.d.ts
index 84505fe..e7e4b59 100644
--- a/src/types/stream.d.ts
+++ b/src/types/stream.d.ts
@@ -10,8 +10,8 @@
 /////////////////////////////////////////////////////////////////////////////////////////////
 
 
-import { CrcCalculator } from "./crc-calculator";
-import { FieldValue } from "./mesg";
+import { CrcCalculator } from "./crc-calculator.js";
+import { FieldValue } from "./mesg.js";
 
 /** Represents a FIT file's bytes and provides sequential read access. */
 export class Stream {