Fifty-five specimens of the polychaete Labrorostratus luteus sp. n. (Arabellidae) were removed from the coelomic cavities of another polychaete species, Haplosyllis spongicola (Syllidae), collected in sponges near Grand Bahama Island. The incidence of parasitism observed in a very large collection of these syllids was approximately 0.2%. Each syllid host contained a single parasite, which almost completely filled the body cavity. Labrorostratus luteus, the third known species of the genus, differs morphologically from Labrorostratus parasiticus primarily in the presence of hooded acicular setae, and from both L. parasiticus and Labrorostratus jonicus in the structure of the mandibles and maxillae.