Two species of thigmotrich ciliates were found in the lamel1ibranch Musculus niger (Gray) taken in the Øresund. One is described as Ancistrum caudatum sp. nov., and the other is idenfied as Hypocomides modiolariae Chatton & Lwoff. The splitting of the genus Ancistrum Maupas into two genera by Raabe (1959) is discussed and found to be questionable. H. modiolariae from the Øresund was found to differ somewhat from the form described by Chatton & Lwoff, but further studies are needed to determine whether these differences should prove to be specific.