diff --git a/src/lib.rs b/src/lib.rs
index 8e6daf9..4b9da92 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -451,3 +451,22 @@ pub fn is_prime(n: u64) -> bool {
     }
     firstfac(n) == n
 }
+
+/// Euler's totient function, the number of primes between 1 and n inclusive that are relatively
+/// prime to n.
+pub fn totient(n: u64) -> u64 {
+    if n <= 0 {
+        return 0;
+    }
+    let mut r = 1u64;
+    let mut prev = 1u64;
+    for i in factors(n) {
+        if i == prev {
+          r *= i;
+        } else {
+            r *= i - 1;
+        }
+        prev = i;
+    }
+    r
+}
\ No newline at end of file
diff --git a/tests/basictests.rs b/tests/basictests.rs
index 70939ff..5644231 100644
--- a/tests/basictests.rs
+++ b/tests/basictests.rs
@@ -1,4 +1,4 @@
-use primes::{factors, factors_uniq, is_prime, PrimeSet, PrimeSetBasics, Sieve, TrialDivision};
+use primes::{factors, factors_uniq, is_prime, totient, PrimeSet, PrimeSetBasics, Sieve, TrialDivision};
 
 #[test]
 fn test_primesetbasics() {
@@ -145,3 +145,14 @@ fn test_sieve() {
 
     assert_eq!(sieved, trialled);
 }
+
+#[test]
+fn test_totient() {
+    assert_eq!(totient(1), 1);
+    assert_eq!(totient(12), 4);
+    assert_eq!(totient(17), 16);
+    assert_eq!(totient(120), 32);
+    assert_eq!(totient(248), 120);
+    assert_eq!(totient(100000000), 40000000);
+    assert_eq!(totient(10493938559900), 4197575423920);
+}
\ No newline at end of file