diff --git a/.gitignore b/.gitignore
index d1c75fb..22ae929 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,7 @@
 
 # Generated by `cargo run --example pick_a_car`
 /pick_a_car.json
+
+# Generated by `cargo mutants`
+/mutants.out
+/mutants.out.old