diff --git a/fs/fs.go b/fs/fs.go
index 2d8206816..32bddac99 100644
--- a/fs/fs.go
+++ b/fs/fs.go
@@ -105,6 +105,9 @@ type ListFser interface {
 	//
 	// This should return ErrDirNotFound (using out.SetError())
 	// if the directory isn't found.
+	//
+	// Fses must support recursion levels of fs.MaxLevel and 1.
+	// They may return ErrorLevelNotSupported otherwise.
 	List(out ListOpts, dir string)
 }