diff --git a/src/chunker/index.js b/src/chunker/index.js
index ec2c494..71ca417 100644
--- a/src/chunker/index.js
+++ b/src/chunker/index.js
@@ -8,7 +8,12 @@ const chunkers = {
 }
 
 module.exports = (type, source, options) => {
-  const chunker = chunkers[type]
+  let chunker
+  if (typeof type === 'function') {
+    chunker = type
+  } else {
+    chunker = chunkers[type]
+  }
 
   if (!chunker) {
     throw errCode(new Error(`Unknkown chunker named ${type}`), 'ERR_UNKNOWN_CHUNKER')
diff --git a/test/chunker-custom.spec.js b/test/chunker-custom.spec.js
new file mode 100644
index 0000000..350ce93
--- /dev/null
+++ b/test/chunker-custom.spec.js
@@ -0,0 +1,36 @@
+/* eslint-env mocha */
+'use strict'
+
+const importer = require('../src')
+
+const chai = require('chai')
+chai.use(require('dirty-chai'))
+const expect = chai.expect
+const IPLD = require('ipld')
+const inMemory = require('ipld-in-memory')
+
+// eslint bug https://github.com/eslint/eslint/issues/12459
+// eslint-disable-next-line require-await
+const iter = async function * () {
+  yield Buffer.from('one')
+  yield Buffer.from('two')
+}
+
+describe('custom chunker', function () {
+  it('keeps custom chunking', async () => {
+    const chunker = source => source
+    const content = iter()
+    const inmem = await inMemory(IPLD)
+    const sizes = [11, 11, 116]
+    const ipld = {
+      put: (node, ...args) => {
+        expect(node.toJSON().size).to.equal(sizes.shift())
+        return inmem.put(node, ...args)
+      }
+    }
+    for await (const part of importer([{ path: 'test', content }], ipld, { chunker })) {
+      expect(part.size).to.equal(116)
+    }
+    expect(sizes).to.be.empty()
+  })
+})