diff --git a/CHANGELOG.md b/CHANGELOG.md
index b2c4c83..43c2ff3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add `prefix` to `string` module.
 - Add `suffix` to `string` module.
 
+### Changed
+
+- Changed overloads of `$mock.union` function.
+
 ### Fixed
 
 - Update `yarn.lock` in order to fix "typescript (18.x)" and "jest (18.x)" jobs in CI.
diff --git a/src/Mock.test.ts b/src/Mock.test.ts
index 1101889..a233a39 100644
--- a/src/Mock.test.ts
+++ b/src/Mock.test.ts
@@ -345,7 +345,23 @@ describe('Mock', () => {
   })
 
   describe('union', () => {
-    const M = union(boolean, number(), string)
+    const M = union(
+      boolean,
+      number(),
+      string,
+      boolean,
+      number(),
+      string,
+      boolean,
+      number(),
+      string,
+      boolean,
+      number(),
+      string,
+      boolean,
+      number(),
+      string,
+    )
 
     it('should return a random value of one of the specified types', () => {
       for (let i = 0; i < 10; i++) {
diff --git a/src/Mock.ts b/src/Mock.ts
index 73a2865..709be4e 100644
--- a/src/Mock.ts
+++ b/src/Mock.ts
@@ -1,3 +1,8 @@
+import { PartialDeep } from '.'
+import { curry, recurse, run } from './function'
+import * as $St from './struct'
+import * as $t from './Type'
+import * as $Y from './Yield'
 import * as Appli from 'fp-ts/Applicative'
 import * as _Apply from 'fp-ts/Apply'
 import * as C from 'fp-ts/Chain'
@@ -24,11 +29,6 @@ import * as RNEA from 'fp-ts/ReadonlyNonEmptyArray'
 import * as RR from 'fp-ts/ReadonlyRecord'
 import * as Se from 'fp-ts/Semigroup'
 import * as t from 'io-ts'
-import { PartialDeep } from '.'
-import { curry, recurse, run } from './function'
-import * as $St from './struct'
-import * as $t from './Type'
-import * as $Y from './Yield'
 
 export const URI = 'Mock'
 
@@ -209,6 +209,86 @@ export const partial = <A>(
     struct,
   ) as any
 
+export function union<A, B, C, D, E, F, G, H, I, J, K, L, M, N, O>(
+  a: Mock<A>,
+  b: Mock<B>,
+  c: Mock<C>,
+  d: Mock<D>,
+  e: Mock<E>,
+  f: Mock<F>,
+  g: Mock<G>,
+  h: Mock<H>,
+  i: Mock<I>,
+  j: Mock<J>,
+  k: Mock<K>,
+  l: Mock<L>,
+  m: Mock<M>,
+  n: Mock<N>,
+  o: Mock<O>,
+): Mock<A | B | C | D | E | F | G | H | I | J | K | L | M | N | O>
+
+export function union<A, B, C, D, E, F, G, H, I, J, K, L, M, N>(
+  a: Mock<A>,
+  b: Mock<B>,
+  c: Mock<C>,
+  d: Mock<D>,
+  e: Mock<E>,
+  f: Mock<F>,
+  g: Mock<G>,
+  h: Mock<H>,
+  i: Mock<I>,
+  j: Mock<J>,
+  k: Mock<K>,
+  l: Mock<L>,
+  m: Mock<M>,
+  n: Mock<N>,
+): Mock<A | B | C | D | E | F | G | H | I | J | K | L | M | N>
+
+export function union<A, B, C, D, E, F, G, H, I, J, K, L, M>(
+  a: Mock<A>,
+  b: Mock<B>,
+  c: Mock<C>,
+  d: Mock<D>,
+  e: Mock<E>,
+  f: Mock<F>,
+  g: Mock<G>,
+  h: Mock<H>,
+  i: Mock<I>,
+  j: Mock<J>,
+  k: Mock<K>,
+  l: Mock<L>,
+  m: Mock<M>,
+): Mock<A | B | C | D | E | F | G | H | I | J | K | L | M>
+
+export function union<A, B, C, D, E, F, G, H, I, J, K, L>(
+  a: Mock<A>,
+  b: Mock<B>,
+  c: Mock<C>,
+  d: Mock<D>,
+  e: Mock<E>,
+  f: Mock<F>,
+  g: Mock<G>,
+  h: Mock<H>,
+  i: Mock<I>,
+  j: Mock<J>,
+  k: Mock<K>,
+  l: Mock<L>,
+): Mock<A | B | C | D | E | F | G | H | I | J | K | L>
+
+export function union<A, B, C, D, E, F, G, H, I, J, K>(
+  a: Mock<A>,
+  b: Mock<B>,
+  c: Mock<C>,
+  d: Mock<D>,
+  e: Mock<E>,
+  f: Mock<F>,
+  g: Mock<G>,
+  h: Mock<H>,
+  i: Mock<I>,
+  j: Mock<J>,
+  k: Mock<K>,
+): Mock<A | B | C | D | E | F | G | H | I | J | K>
+
 export function union<A, B, C, D, E, F, G, H, I, J>(
   a: Mock<A>,
   b: Mock<B>,