Excavations conducted in Naj Tunich, Peten, Guatemala encountered a number of slabs of speleothem curtains that were used as altars. Two of these contained bat skeletons suggesting that bats had been sacrificed as part of ceremonies carried out in the cave. A review of the archaeological literature documents that remains of bats has been reported in burials, caches, and constructions. Naj Tunich, however, is the first instance of sacrifice occurring in a cave which raises the problem of distinguishing between cultural as opposed to natural deposition. A series of propositions are advanced for dealing with the issue.