diff --git a/index.js b/index.js
index c1df2d3..bbc8afb 100644
--- a/index.js
+++ b/index.js
@@ -24,7 +24,7 @@ const getTypeMismatchError = ({ filename, expected, actual }) => {
 const cssModuleToInterface = (cssModuleKeys) => {
   const interfaceFields = cssModuleKeys
     .sort()
-    .map(key => `  '${key}': string;`)
+    .map(key => `  readonly '${key}': string;`)
     .join('\n');
 
   return `interface CssExports {\n${interfaceFields}\n}`;
diff --git a/test/emit-declaration/__snapshots__/emit-declaration.test.js.snap b/test/emit-declaration/__snapshots__/emit-declaration.test.js.snap
index 1164931..c1f0bf6 100644
--- a/test/emit-declaration/__snapshots__/emit-declaration.test.js.snap
+++ b/test/emit-declaration/__snapshots__/emit-declaration.test.js.snap
@@ -4,10 +4,10 @@ exports[`Can emit valid declaration with sourceMap 1`] = `
 "// This file is automatically generated.
 // Please do not change this file!
 interface CssExports {
-  'composedClass': string;
-  'otherClass': string;
-  'someClass': string;
-  'validClass': string;
+  readonly 'composedClass': string;
+  readonly 'otherClass': string;
+  readonly 'someClass': string;
+  readonly 'validClass': string;
 }
 export const cssExports: CssExports;
 export default cssExports;
@@ -18,10 +18,10 @@ exports[`Can emit valid declaration without sourceMaps 1`] = `
 "// This file is automatically generated.
 // Please do not change this file!
 interface CssExports {
-  'composedClass': string;
-  'otherClass': string;
-  'someClass': string;
-  'validClass': string;
+  readonly 'composedClass': string;
+  readonly 'otherClass': string;
+  readonly 'someClass': string;
+  readonly 'validClass': string;
 }
 export const cssExports: CssExports;
 export default cssExports;
diff --git a/test/verify-invalid-declaration/__snapshots__/verify-invalid-declaration.test.js.snap b/test/verify-invalid-declaration/__snapshots__/verify-invalid-declaration.test.js.snap
index 82df075..37d7a7a 100644
--- a/test/verify-invalid-declaration/__snapshots__/verify-invalid-declaration.test.js.snap
+++ b/test/verify-invalid-declaration/__snapshots__/verify-invalid-declaration.test.js.snap
@@ -6,9 +6,10 @@ exports[`Can error on invalid declaration 1`] = `
    // This file is automatically generated.
    // Please do not change this file!
    interface CssExports {
-     'classInBothFiles': string;
+ -   'classInBothFiles': string;
  -   'classInTypeScriptFile': string;
- +   'classInCssFile': string;
+ +   readonly 'classInBothFiles': string;
+ +   readonly 'classInCssFile': string;
    }
    export const cssExports: CssExports;
    export default cssExports;
diff --git a/test/verify-valid-declaration/index.css.d.ts b/test/verify-valid-declaration/index.css.d.ts
index 9dfd5bc..7df58d7 100644
--- a/test/verify-valid-declaration/index.css.d.ts
+++ b/test/verify-valid-declaration/index.css.d.ts
@@ -1,11 +1,11 @@
 // This file is automatically generated.
 // Please do not change this file!
 interface CssExports {
-  'hyphened-classname': string;
-  'otherClass': string;
-  'someClass': string;
-  'underscored_classname': string;
-  'validClass': string;
+  readonly 'hyphened-classname': string;
+  readonly 'otherClass': string;
+  readonly 'someClass': string;
+  readonly 'underscored_classname': string;
+  readonly 'validClass': string;
 }
 export const cssExports: CssExports;
 export default cssExports;