diff --git a/mutaml.opam b/mutaml.opam
index ec5702b..ccd246a 100644
--- a/mutaml.opam
+++ b/mutaml.opam
@@ -37,6 +37,7 @@ x-maintenance-intent: ["(latest)"]
 build: [
   ["dune" "subst"] {dev}
   [
+    "MUTAML_DIFF_COMMAND='gdiff -u'" {os = "freebsd"}
     "dune"
     "build"
     "-p"
diff --git a/mutaml.opam.template b/mutaml.opam.template
index 892c85d..9f9ac75 100644
--- a/mutaml.opam.template
+++ b/mutaml.opam.template
@@ -1,6 +1,7 @@
 build: [
   ["dune" "subst"] {dev}
   [
+    "MUTAML_DIFF_COMMAND='gdiff -u'" {os = "freebsd"}
     "dune"
     "build"
     "-p"