Dwarf razor clams (Ensis minor) in the Gulf of Mexico are known to be infected with plerocercoid larvae of a tetraphyllidean tapeworm. Here, we show that these larvae live unencysted in the intestinal lumen of the clam. Morphologically, the larvae are similar to (although significantly larger than) tapeworm larvae previously described living in the gut of amphioxus (Branchiostoma floridae) from the same habitat. Sequence data from the D2 region of the 28S rDNA from clam-infecting larvae were identical to the sequence of A. brevissime isolated as larvae from amphioxus and as adults from a stingray (Dasyatis say). The sequence data leave little doubt that the dwarf razor clam and amphioxus are alternative intermediate hosts in the life cycle of Acanthobothrium brevissime.