Filter characteristics have been determined and compared in ciliary and mucus-net filter feeders. The ciliary feeders include the polychaete Sabella penicillus, the brachiopod Terebratulina retuso, the marine bivalves Monia squama, Cardiurn glaucum, and Petricola pholadiformis, and the freshwater bivalves Dreissena polymorpha, Unio pictamrn, and Anodonta cygnea. The mucus-net feeders are the polychaete Chaetopterus variopedatus, the gastropod Crepidula fomicata, and the ascidians Sfyela clava, Ciona intestinalis, Ascidia virginia, Ascidia obliqua, and Ascidia mentula. Efficiencies of particle retention as a function of particle size was determined by counting of particles in samples of inhalant and exhalant water. The lower threshold for efficient particle retention varied from about 6 p m in T. retuso to about 1 p n in D. polyrnorpha. Mucus nets efficiently retained particles down to 1 to 2 pm. Filter feeding is characterized by processing of water at low pressures (5 l mm H,O). Mechanisms of water processing and particle retention in brachiopods and bivalves are compared. It is concluded that laminar flow of through-currents and surface-currents in brachiopods is consistent with the hypothesis of capture of suspended particles by means of viscous forces acting upon the particles in the zone of contact between the 2 flow systems.