diff --git a/.travis.yml b/.travis.yml
index 0e4e989..6a5a6db 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,17 @@
 sudo: false
-
 language: rust
 
-rust:
-  - nightly
+matrix:
+  include:
+    - rust: stable
+    - rust: beta
+    - rust: nightly
+      env:
+      - FEATURES="i128"
+      - BUILD_BENCH="true"
+
+script:
+  - cargo build --verbose --features "$FEATURES"
+  - cargo test --verbose --features "$FEATURES"
+  - if [ "$BUILD_BENCH" == "true" ]; then cargo bench --verbose --no-run --features "$FEATURES"; fi
+
diff --git a/Cargo.toml b/Cargo.toml
index db6516d..6eb09b2 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,3 +9,6 @@ documentation = "https://github.com/dtolnay/itoa"
 categories = ["value-formatting"]
 readme = "README.md"
 exclude = ["performance.png"]
+
+[features]
+i128 = []
diff --git a/README.md b/README.md
index 9e202e9..524957b 100644
--- a/README.md
+++ b/README.md
@@ -41,8 +41,9 @@ fn write<W: io::Write, V: itoa::Integer>(writer: W, value: V) -> io::Result<usiz
 ```
 
 where `itoa::Integer` is implemented for `i8`, `u8`, `i16`, `u16`, `i32`, `u32`,
-`i64`, `u64`, `isize` and `usize`. The return value gives the number of bytes
-written.
+`i64`, `u64`, `i128`, `u128`, `isize` and `usize`. 128-bit integer support is
+only available with the nightly compiler when the `i128` feature is enabled for
+this crate. The return value gives the number of bytes written.
 
 ## Dependency
 
diff --git a/benches/bench.rs b/benches/bench.rs
index 5912fc2..0cdd7b4 100644
--- a/benches/bench.rs
+++ b/benches/bench.rs
@@ -1,3 +1,4 @@
+#![cfg_attr(feature = "i128", feature(i128_type, i128))]
 #![feature(test)]
 #![allow(non_snake_case)]
 
@@ -5,15 +6,21 @@ extern crate itoa;
 extern crate test;
 
 macro_rules! benches {
-    ($($name:ident($value:expr),)*) => {
+    (
+        $(
+            $(#[$attr:meta])*
+            $name:ident($value:expr)
+        ),*
+    ) => {
         mod bench_itoa {
             use test::{Bencher, black_box};
             $(
+                $(#[$attr])*
                 #[bench]
                 fn $name(b: &mut Bencher) {
                     use itoa;
 
-                    let mut buf = Vec::with_capacity(20);
+                    let mut buf = Vec::with_capacity(40);
 
                     b.iter(|| {
                         buf.clear();
@@ -26,11 +33,12 @@ macro_rules! benches {
         mod bench_fmt {
             use test::{Bencher, black_box};
             $(
+                $(#[$attr])*
                 #[bench]
                 fn $name(b: &mut Bencher) {
                     use std::io::Write;
 
-                    let mut buf = Vec::with_capacity(20);
+                    let mut buf = Vec::with_capacity(40);
 
                     b.iter(|| {
                         buf.clear();
@@ -42,11 +50,16 @@ macro_rules! benches {
     }
 }
 
-benches!(
-    bench_0u64(0u64),
-    bench_HALFu64(<u32>::max_value() as u64),
-    bench_MAXu64(<u64>::max_value()),
+benches!{
+    bench_u64_0(0u64),
+    bench_u64_half(<u32>::max_value() as u64),
+    bench_u64_max(<u64>::max_value()),
 
-    bench_0i16(0i16),
-    bench_MINi16(<i16>::min_value()),
-);
+    bench_i16_0(0i16),
+    bench_i16_min(<i16>::min_value()),
+
+    #[cfg(feature = "i128")]
+    bench_u128_0(0u128),
+    #[cfg(feature = "i128")]
+    bench_u128_max(<u128>::max_value())
+}
diff --git a/src/lib.rs b/src/lib.rs
index 9f8b030..b999365 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -8,6 +8,8 @@
 
 #![doc(html_root_url = "https://docs.rs/itoa/0.3.3")]
 
+#![cfg_attr(feature = "i128", feature(i128_type, i128))]
+
 use std::{io, mem, ptr, slice};
 
 #[inline]
@@ -30,7 +32,7 @@ const DEC_DIGITS_LUT: &'static[u8] =
       6061626364656667686970717273747576777879\
       8081828384858687888990919293949596979899";
 
-const MAX_LEN: usize = 20;  // Tie between i64::MIN (including minus sign) and u64::MAX
+const MAX_LEN: usize = 40; // i128::MIN (including minus sign)
 
 // Adaptation of the original implementation at
 // https://github.com/rust-lang/rust/blob/b8214dc6c6fc20d0a660fb5700dca9ebf51ebe89/src/libcore/fmt/num.rs#L188-L266
@@ -115,3 +117,5 @@ impl_Integer!(isize, usize as u16);
 impl_Integer!(isize, usize as u32);
 #[cfg(target_pointer_width = "64")]
 impl_Integer!(isize, usize as u64);
+#[cfg(feature = "i128")]
+impl_Integer!(i128, u128 as u128);
diff --git a/tests/test.rs b/tests/test.rs
index 0a33f80..c6e26de 100644
--- a/tests/test.rs
+++ b/tests/test.rs
@@ -1,13 +1,20 @@
+#![cfg_attr(feature = "i128", feature(i128_type, i128))]
 #![allow(non_snake_case)]
 
 extern crate itoa;
 
 macro_rules! test {
-    ($($name:ident($value:expr, $expected:expr),)*) => {
+    (
         $(
+            $(#[$attr:meta])*
+            $name:ident($value:expr, $expected:expr)
+        ),*
+    ) => {
+        $(
+            $(#[$attr])*
             #[test]
             fn $name() {
-                let mut buf = [b'\0'; 20];
+                let mut buf = [b'\0'; 40];
                 let len = itoa::write(&mut buf[..], $value).unwrap();
                 assert_eq!(&buf[0..len], $expected.as_bytes());
             }
@@ -15,12 +22,19 @@ macro_rules! test {
     }
 }
 
-test!(
-    test_0u64(0u64, "0"),
-    test_HALFu64(<u32>::max_value() as u64, "4294967295"),
-    test_MAXu64(<u64>::max_value(), "18446744073709551615"),
-    test_MINi64(<i64>::min_value(), "-9223372036854775808"),
+test!{
+    test_u64_0(0u64, "0"),
+    test_u64_half(<u32>::max_value() as u64, "4294967295"),
+    test_u64_max(<u64>::max_value(), "18446744073709551615"),
+    test_i64_min(<i64>::min_value(), "-9223372036854775808"),
 
-    test_0i16(0i16, "0"),
-    test_MINi16(<i16>::min_value(), "-32768"),
-);
+    test_i16_0(0i16, "0"),
+    test_i16_min(<i16>::min_value(), "-32768"),
+
+    #[cfg(feature = "i128")]
+    test_u128_0(0u128, "0"),
+    #[cfg(feature = "i128")]
+    test_u128_max(<u128>::max_value(), "340282366920938463463374607431768211455"),
+    #[cfg(feature = "i128")]
+    test_i128_min(<i128>::min_value(), "-170141183460469231731687303715884105728")
+}