diff --git a/README.md b/README.md
index 2842358..d6f0257 100644
--- a/README.md
+++ b/README.md
@@ -27,7 +27,7 @@ import Replicate from "replicate";
 
 const replicate = new Replicate({
   // get your token from https://replicate.com/account
-  auth: process.env.REPLICATE_API_TOKEN,
+  auth: "my api token", // defaults to process.env.REPLICATE_API_TOKEN
 });
 ```
 
@@ -124,18 +124,14 @@ and pass it to the `fetch` option in the constructor.
 import Replicate from "replicate";
 import fetch from "cross-fetch";
 
-const replicate = new Replicate({
-  // get your token from https://replicate.com/account
-  auth: process.env.REPLICATE_API_TOKEN,
-  fetch: fetch,
-});
+const replicate = new Replicate({ fetch });
 ```
 
 You can override the `fetch` property to add custom behavior to client requests,
 such as injecting headers or adding log statements.
 
 ```js
-client.fetch = (url, options) => {
+replicate.fetch = (url, options) => {
   const headers = new Headers(options && options.headers);
   headers.append("X-Custom-Header", "some value");
 
diff --git a/index.d.ts b/index.d.ts
index 50ebd1c..32f279a 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -69,8 +69,8 @@ declare module 'replicate' {
   }
 
   export default class Replicate {
-    constructor(options: {
-      auth: string;
+    constructor(options?: {
+      auth?: string;
       userAgent?: string;
       baseUrl?: string;
       fetch?: Function;
diff --git a/index.js b/index.js
index 9662235..c6a2cc2 100644
--- a/index.js
+++ b/index.js
@@ -32,13 +32,13 @@ class Replicate {
    * Create a new Replicate API client instance.
    *
    * @param {object} options - Configuration options for the client
-   * @param {string} options.auth - Required. API access token
+   * @param {string} options.auth - API access token. Defaults to the `REPLICATE_API_TOKEN` environment variable.
    * @param {string} options.userAgent - Identifier of your app
    * @param {string} [options.baseUrl] - Defaults to https://api.replicate.com/v1
    * @param {Function} [options.fetch] - Fetch function to use. Defaults to `globalThis.fetch`
    */
   constructor(options = {}) {
-    this.auth = options.auth;
+    this.auth = options.auth || process.env.REPLICATE_API_TOKEN;
     this.userAgent =
       options.userAgent || `replicate-javascript/${packageJSON.version}`;
     this.baseUrl = options.baseUrl || 'https://api.replicate.com/v1';
diff --git a/index.test.ts b/index.test.ts
index a560da1..c35b41f 100644
--- a/index.test.ts
+++ b/index.test.ts
@@ -51,9 +51,12 @@ describe('Replicate client', () => {
     });
 
     test('Does not throw error if auth token is not provided', () => {
+      process.env.REPLICATE_API_TOKEN = 'test-token';
+
       expect(() => {
-        // @ts-expect-error
-        new Replicate();
+        const clientWithImplicitAuth = new Replicate();
+
+        expect(clientWithImplicitAuth.auth).toBe('test-token');
       }).not.toThrow();
     });