Only one species of Tanaidacea, Expina typica , has been hitherto reported as an endosymbiont; it was recovered from the body cavity of deep-sea holothurians. During a survey of the deepsea benthic community in the Florida Straits off the Bahamas, Terebellatanais floridanus , a new genus and species of a tanaidomorphan tanaidacean was recovered at a depth of 545 m from the oral cavity of the terebellid polychaete Biremis blandi . Terebellatanais gen. nov. is tentatively assigned to the family Mirandotanaidae, as it appears to have affinities with Mirandotanais and Pooreotanais but also with Expina . It is distinguished from these genera by a unique combination of characters, including four antennular and antennal articles, a naked endite of the maxillipedal basis, hook-like chelipeds, the armature of the pereopods, and characters of the mouthparts. Most of the specimens of T . floridanus examined are mancas, but some were distinctly larger and with a different development of the last pereopod, thus suggesting that at least two manca stages are represented. The occurrence of the new genus and species within the oral cavity of B . blandi , its peculiar morphology, and the absence of fully grown adults, suggest that T . floridanus may be a symbiont of Biremis blandi at least during part of its development. Whether this relationship is commensal or parasitic remains to be determined.