diff --git a/src/libstd/bitv.rs b/src/libstd/bitv.rs
index 91e9a1dc94062..901430a279b4f 100644
--- a/src/libstd/bitv.rs
+++ b/src/libstd/bitv.rs
@@ -19,37 +19,42 @@ export to_str;
 export eq_vec;
 export methods;
 
+/// a mask that has a 1 for each defined bit in a small_bitv, assuming n bits
+#[inline(always)]
+fn small_mask(nbits: uint) -> u32 {
+    (1 << nbits) - 1
+}
+
 struct small_bitv {
+    /// only the lowest nbits of this value are used. the rest is undefined.
     let mut bits: u32;
     new(bits: u32) { self.bits = bits; }
     priv {
         #[inline(always)]
-        fn bits_op(right_bits: u32, f: fn(u32, u32) -> u32) -> bool {
+        fn bits_op(right_bits: u32, nbits: uint, f: fn(u32, u32) -> u32)
+                                                                     -> bool {
+            let mask = small_mask(nbits);
             let old_b: u32 = self.bits;
             let new_b = f(old_b, right_bits);
             self.bits = new_b;
-            old_b != new_b
+            mask & old_b != mask & new_b
         }
     }
     #[inline(always)]
-    fn union(s: &small_bitv) -> bool {
-        self.bits_op(s.bits, |u1, u2| { u1 | u2 })
+    fn union(s: &small_bitv, nbits: uint) -> bool {
+        self.bits_op(s.bits, nbits, |u1, u2| u1 | u2)
     }
     #[inline(always)]
-    fn intersect(s: &small_bitv) -> bool {
-        self.bits_op(s.bits, |u1, u2| { u1 & u2 })
+    fn intersect(s: &small_bitv, nbits: uint) -> bool {
+        self.bits_op(s.bits, nbits, |u1, u2| u1 & u2)
     }
     #[inline(always)]
-    fn become(s: &small_bitv) -> bool {
-        let old = self.bits;
-        self.bits = s.bits;
-        old != self.bits
+    fn become(s: &small_bitv, nbits: uint) -> bool {
+        self.bits_op(s.bits, nbits, |_u1, u2| u2)
     }
     #[inline(always)]
-    fn difference(s: &small_bitv) -> bool {
-        let old = self.bits;
-        self.bits &= !s.bits;
-        old != self.bits
+    fn difference(s: &small_bitv, nbits: uint) -> bool {
+        self.bits_op(s.bits, nbits, |u1, u2| u1 ^ u2)
     }
     #[inline(always)]
     pure fn get(i: uint) -> bool {
@@ -65,38 +70,66 @@ struct small_bitv {
         }
     }
     #[inline(always)]
-    fn equals(b: &small_bitv) -> bool { self.bits == b.bits }
+    fn equals(b: &small_bitv, nbits: uint) -> bool {
+        let mask = small_mask(nbits);
+        mask & self.bits == mask & b.bits
+    }
     #[inline(always)]
     fn clear() { self.bits = 0; }
     #[inline(always)]
     fn set_all() { self.bits = !0; }
     #[inline(always)]
-    fn is_true() -> bool { self.bits == !0 }
+    fn is_true(nbits: uint) -> bool {
+        small_mask(nbits) & !self.bits == 0
+    }
     #[inline(always)]
-    fn is_false() -> bool { self.bits == 0 }
+    fn is_false(nbits: uint) -> bool {
+        small_mask(nbits) & self.bits == 0
+    }
     #[inline(always)]
     fn invert() { self.bits = !self.bits; }
 }
 
+/**
+ * a mask that has a 1 for each defined bit in the nth element of a big_bitv,
+ * assuming n bits.
+ */
+#[inline(always)]
+fn big_mask(nbits: uint, elem: uint) -> uint {
+    let rmd = nbits % uint_bits;
+    let nelems = nbits/uint_bits + if rmd == 0 {0} else {1};
+
+    if elem < nelems - 1 || rmd == 0 {
+        !0
+    } else {
+        (1 << rmd) - 1
+    }
+}
+
 struct big_bitv {
-// only mut b/c of clone and lack of other constructor
+    // only mut b/c of clone and lack of other constructor
     let mut storage: ~[mut uint];
     new(-storage: ~[mut uint]) {
         self.storage <- storage;
     }
     priv {
         #[inline(always)]
-        fn process(b: &big_bitv, op: fn(uint, uint) -> uint) -> bool {
+        fn process(b: &big_bitv, nbits: uint, op: fn(uint, uint) -> uint)
+                                                                     -> bool {
             let len = b.storage.len();
             assert (self.storage.len() == len);
             let mut changed = false;
             do uint::range(0, len) |i| {
-                let w0 = self.storage[i];
-                let w1 = b.storage[i];
-                let w = op(w0, w1);
-                if w0 != w unchecked { changed = true; self.storage[i] = w; };
+                let mask = big_mask(nbits, i);
+                let w0 = self.storage[i] & mask;
+                let w1 = b.storage[i] & mask;
+                let w = op(w0, w1) & mask;
+                if w0 != w unchecked {
+                    changed = true;
+                    self.storage[i] = w;
+                }
                 true
-            };
+            }
             changed
         }
     }
@@ -112,15 +145,21 @@ struct big_bitv {
     #[inline(always)]
     fn invert() { for self.each_storage() |w| { w = !w } }
     #[inline(always)]
-    fn union(b: &big_bitv)     -> bool { self.process(b, lor) }
+    fn union(b: &big_bitv, nbits: uint) -> bool {
+        self.process(b, nbits, lor)
+    }
     #[inline(always)]
-    fn intersect(b: &big_bitv) -> bool { self.process(b, land) }
+    fn intersect(b: &big_bitv, nbits: uint) -> bool {
+        self.process(b, nbits, land)
+    }
     #[inline(always)]
-    fn become(b: &big_bitv)    -> bool { self.process(b, right) }
+    fn become(b: &big_bitv, nbits: uint) -> bool {
+        self.process(b, nbits, right)
+    }
     #[inline(always)]
-    fn difference(b: &big_bitv) -> bool {
+    fn difference(b: &big_bitv, nbits: uint) -> bool {
         self.invert();
-        let b = self.intersect(b);
+        let b = self.intersect(b, nbits);
         self.invert();
         b
     }
@@ -140,10 +179,13 @@ struct big_bitv {
                  else { self.storage[w] & !flag };
     }
     #[inline(always)]
-    fn equals(b: &big_bitv) -> bool {
+    fn equals(b: &big_bitv, nbits: uint) -> bool {
         let len = b.storage.len();
         for uint::iterate(0, len) |i| {
-            if self.storage[i] != b.storage[i] { return false; }
+            let mask = big_mask(nbits, i);
+            if mask & self.storage[i] != mask & b.storage[i] {
+                return false;
+            }
         }
     }
 }
@@ -163,8 +205,10 @@ struct bitv {
           self.rep = small(~small_bitv(if init {!0} else {0}));
         }
         else {
-          let s = to_mut(from_elem(nbits / uint_bits + 1,
-                                        if init {!0} else {0}));
+          let nelems = nbits/uint_bits +
+                       if nbits % uint_bits == 0 {0} else {1};
+          let elem = if init {!0} else {0};
+          let s = to_mut(from_elem(nelems, elem));
           self.rep = big(~big_bitv(s));
         };
     }
@@ -182,20 +226,20 @@ struct bitv {
             match self.rep {
               small(s) => match other.rep {
                 small(s1) => match op {
-                  union      => s.union(s1),
-                  intersect  => s.intersect(s1),
-                  assign     => s.become(s1),
-                  difference => s.difference(s1)
+                  union      => s.union(s1,      self.nbits),
+                  intersect  => s.intersect(s1,  self.nbits),
+                  assign     => s.become(s1,     self.nbits),
+                  difference => s.difference(s1, self.nbits)
                 },
                 big(s1) => self.die()
               },
               big(s) => match other.rep {
                 small(_) => self.die(),
                 big(s1) => match op {
-                  union      => s.union(s1),
-                  intersect  => s.intersect(s1),
-                  assign     => s.become(s1),
-                  difference => s.difference(s1)
+                  union      => s.union(s1,      self.nbits),
+                  intersect  => s.intersect(s1,  self.nbits),
+                  assign     => s.become(s1,     self.nbits),
+                  difference => s.difference(s1, self.nbits)
                 }
               }
             }
@@ -280,11 +324,11 @@ struct bitv {
       if self.nbits != v1.nbits { return false; }
       match self.rep {
         small(b) => match v1.rep {
-          small(b1) => b.equals(b1),
+          small(b1) => b.equals(b1, self.nbits),
           _ => false
         },
         big(s) => match v1.rep {
-          big(s1) => s.equals(s1),
+          big(s1) => s.equals(s1, self.nbits),
           small(_) => return false
         }
       }
@@ -330,7 +374,7 @@ struct bitv {
     #[inline(always)]
     fn is_true() -> bool {
       match self.rep {
-        small(b) => b.is_true(),
+        small(b) => b.is_true(self.nbits),
         _ => {
           for self.each() |i| { if !i { return false; } }
           true
@@ -351,7 +395,7 @@ struct bitv {
 
     fn is_false() -> bool {
       match self.rep {
-        small(b) => b.is_false(),
+        small(b) => b.is_false(self.nbits),
         big(_) => {
           for self.each() |i| { if i { return false; } }
           true
@@ -740,6 +784,33 @@ mod tests {
         let v1 = bitv(110u, false);
         assert !v0.equal(v1);
     }
+
+    #[test]
+    fn test_equal_sneaky_small() {
+        let a = bitv::bitv(1, false);
+        a.set(0, true);
+
+        let b = bitv::bitv(1, true);
+        b.set(0, true);
+
+        assert a.equal(b);
+    }
+
+    #[test]
+    fn test_equal_sneaky_big() {
+        let a = bitv::bitv(100, false);
+        for uint::range(0, 100) |i| {
+            a.set(i, true);
+        }
+
+        let b = bitv::bitv(100, true);
+        for uint::range(0, 100) |i| {
+            b.set(i, true);
+        }
+
+        assert a.equal(b);
+    }
+
 }
 
 //