diff --git a/src/librustc/mir/repr.rs b/src/librustc/mir/repr.rs
index b6e86077ccf..93e1cffa6bc 100644
--- a/src/librustc/mir/repr.rs
+++ b/src/librustc/mir/repr.rs
@@ -184,7 +184,7 @@ pub fn index(self) -> usize {
 
 impl Debug for BasicBlock {
     fn fmt(&self, fmt: &mut Formatter) -> Result<(), Error> {
-        write!(fmt, "BB({})", self.0)
+        write!(fmt, "bb{}", self.0)
     }
 }