Not in my fixtures, it looks like their diameter is different. Your best bet is to get yourself over to HD where they have either the phillips 3 pack or Ecosmart 3 packs for $22 or $12, which is not as good as the costco deal but better than I've seen in a while. If it's used in a pot, the length of a PAR16 is longer but seems to fit in most fixtures, as long as you don't need to tilt it much.
Else people have been talking about getting E26 to MR16 adapter but that adds significant length to the bulb, too.