diff --git a/formatter/formatter.go b/formatter/formatter.go
index 5d331c09..02b8c116 100644
--- a/formatter/formatter.go
+++ b/formatter/formatter.go
@@ -385,7 +385,7 @@ func (f *formatter) FormatDefinition(def *ast.Definition, extend bool) {
 	}
 
 	if len(def.Interfaces) != 0 {
-		f.WriteWord("implements").WriteWord(strings.Join(def.Interfaces, ", "))
+		f.WriteWord("implements").WriteWord(strings.Join(def.Interfaces, " & "))
 	}
 
 	f.FormatDirectiveList(def.Directives)