Rates of uptake of organic and inorganic suspended particulate material by suspension feeding bivalve molluscs have been studied since the nineteenth century (see Viallanes, 1892) . Studies have investigated molluscan feeding mechanisms, filtration capabilities, food assimilation and metabolism. The bulk of this work has been directed towards the mussel Mytilus edulis Linnaeus and the oyster Crassostrea virginica Gmelin (see reviews by Galtsoff, 1964; Ali, 1970; Jorgensen, 1975; 1976; and Winter, 1978). More recently the growth of mussels, oysters, clams and scallops has been quantitatively studied in controlled systems to assess the biological potential of these molluscs in an aquaculture setting (Hartman et al., 1973; Tenore & Dunstan, 1973; Tenore et al., 1973; Kirby-Smith & Barber, 1975; Walne & Spencer, 1974; Epifanio & Ewart, 1977; Winter, 1978). Direct and indirect methods for estimating filtration rates of molluscs (Ali, 1970) have yielded such a wide range of results that the validity of comparisons between different studies is sometimes questionable . Variables such as the mollusc species and size, as well as the nature and concentration of the test suspension contribute to differences in observed filtration rates. The present study simultaneously examined three mytilid speeies of comparable size (85-125 mm shell length) . Each mussel species was tested under the same conditions for its ability to filter and assimilate the unicellular algae Dunaliella primolecta Butcher at suspensions of 5, 12, 25 and 50 x 106cells/1. Filtration rates and food assimilation were determined in test chambers incorporating a new flow-through design which eliminated the possibility of recirculation of the algae test suspension. In previous studies recirculation of the test suspension has occasionally resulted in the under-estimation of filtration rates. Growth comparisons were made among individuals of each species held under the same set of environmental conditions. Two of the mussel species examined in this study, Mytilus edulis L. and Perna canaliculus Gmelin, are presently under cultivation as a human food source. The third species, Mytilus catifornianus Conrad, may be viewed as a potential candidate for aquaculture due to its size, abundance and value as a source of protein. The California mussel, M. catifornianus, is found along the west coast of North America from the Aleutian Islands to Baja California (Soot-Ryen, 1955). The bay mussel, M. edutis , is widespread in the northern and southern hemispheres (Stubbings, 1954). The green-lipped mussel, P. canaliculus, is found throughout New Zealand waters where it colonizes both exposed rocky coasts and quiet bays (Morton & Miller, 1968; Paine, 1971).