diff --git a/components/selectors/bloom.rs b/components/selectors/bloom.rs
index 9475676ec71..2b96e4f51f8 100644
--- a/components/selectors/bloom.rs
+++ b/components/selectors/bloom.rs
@@ -6,6 +6,7 @@
 //! for selector matching.
 
 use fnv::FnvHasher;
+use std::fmt::{self, Debug};
 use std::hash::{Hash, Hasher};
 
 // The top 8 bits of the 32-bit hash value are not used by the bloom filter.
@@ -141,6 +142,18 @@ impl<S> CountingBloomFilter<S> where S: BloomStorage {
     }
 }
 
+impl<S> Debug for CountingBloomFilter<S> where S: BloomStorage {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let mut slots_used = 0;
+        for i in 0..ARRAY_SIZE {
+            if !self.storage.slot_is_empty(i) {
+                slots_used += 1;
+            }
+        }
+        write!(f, "BloomFilter({}/{})", slots_used, ARRAY_SIZE)
+    }
+}
+
 pub trait BloomStorage : Clone + Default {
     fn slot_is_empty(&self, index: usize) -> bool;
     fn adjust_slot(&mut self, index: usize, increment: bool);