Plerocercoid larvae of a tapeworm are frequently found in the hindgut lumen of the Florida amphioxus (Branchiostoma floridae) in central west Florida. About three-quarters of the adult amphioxus are parasitized. On average, each adult amphioxus hosts about five tapeworm larvae. The residence time of the parasites in the amphioxus gut appears to be on the order of several months, which is considerably shorter than the potential lifetime of the host. The living larvae range in length (when fully extended) from 300 to 850 μm and are approximately cone-shaped, tapering to a point posteriorly and bearing a single large sucker anteriorly. Toward the anterior end of the body are four bothridia, each indented by three loculi plus an inconspicuous accessory sucker, but not provided with any bothridial hooks. The larvae initiate the early stages of hook growth when they are cultured for a few days in urea-saline (mimicking the gut fluid of the definitive host, which is an elasmobranch). The tapeworm larvae are identifiable to genus and species on the basis of correspondences between their nuclear ribosomal DNA genes and those of adult specimens of Acanthobothrium brevissime recovered from the spiral valve of a stingray from the same environment.