diff --git a/src/end-to-end.test.ts b/src/end-to-end.test.ts
index 1f23d4c..1be77eb 100644
--- a/src/end-to-end.test.ts
+++ b/src/end-to-end.test.ts
@@ -339,7 +339,7 @@ testCases(endToEnd, code => code)('end-to-end tests', [
      4`,
     either.makeRight('10'),
   ],
-  [`{ f: _ => 1 + 1 }.f(whatever)`, either.makeRight('2')],
+  [`{ f: _ => 5 % 3 }.f(whatever)`, either.makeRight('2')],
   [
     `{
       one: 1
diff --git a/src/language/semantics/prelude.ts b/src/language/semantics/prelude.ts
index d592d8a..5e8266f 100644
--- a/src/language/semantics/prelude.ts
+++ b/src/language/semantics/prelude.ts
@@ -21,4 +21,5 @@ export const prelude = makeObjectNode({
   '-': integer.subtract,
   '<': integer.less_than,
   '>': integer.greater_than,
+  '%': natural_number.modulo,
 })
diff --git a/src/language/semantics/stdlib/natural-number.ts b/src/language/semantics/stdlib/natural-number.ts
index 4e35f34..52a264b 100644
--- a/src/language/semantics/stdlib/natural-number.ts
+++ b/src/language/semantics/stdlib/natural-number.ts
@@ -75,4 +75,44 @@ export const natural_number = {
           : 'false',
       ),
   ),
+  modulo: preludeFunction(
+    ['natural_number', 'modulo'],
+    {
+      parameter: types.naturalNumber,
+      return: makeFunctionType('', {
+        parameter: types.naturalNumber,
+        return: types.naturalNumber,
+      }),
+    },
+    number2 =>
+      either.makeRight(
+        makeFunctionNode(
+          {
+            parameter: types.naturalNumber,
+            return: types.naturalNumber,
+          },
+          serializeOnceAppliedFunction(['natural_number', 'modulo'], number2),
+          option.none,
+          number1 => {
+            if (
+              typeof number1 !== 'string' ||
+              !types.naturalNumber.isAssignableFrom(
+                makeUnionType('', [number1]),
+              ) ||
+              typeof number2 !== 'string' ||
+              !types.naturalNumber.isAssignableFrom(
+                makeUnionType('', [number2]),
+              )
+            ) {
+              return either.makeLeft({
+                kind: 'panic',
+                message: 'numbers must be atoms',
+              })
+            } else {
+              return either.makeRight(String(BigInt(number1) % BigInt(number2)))
+            }
+          },
+        ),
+      ),
+  ),
 }