When individuals of 5–811 mg dry tissue weight were fed 100 × 10 6 cells l −1 Isochrysis galbana for 24 h in Winter's (1973) apparatus the filtration rate (ml h −1 oyster −1) increased with increasing body size following the allometric equation FR=1.2W 0.73 where W is dry tissue weight in mg. Filtration rate at three different algal densities decreased with increasing densities, showing that C. gigas can regulate the filtration rate in that concentration range. The amount of water filtered free of particles was in the ratio 1:2:3 with decreasing concentrations from 100 to 75 and 50 × 10 6 cells l −1 I. galbana. Consequently, the amount of algae removed was more or less constant at all three concentrations. Assimilation efficiency was found to be more or less constant, not being significantly dependent on body size or algal concentration. Filtration rates of oyster larvae were determined by measuring the decrease in algal concentration with time. When fed 50 × 10 6 cells l −1 of both I. galbana and Chaectoceros calcitrans filtration rates increased from about 0.5 to 100 μl h −1 larva −1 during larval development. Filtration rates increased discontinuously with increasing body size; larvae from about 77–120 μm shell length filtered less actively than larvae > 120 μm shell length; a marked drop in filtration activity was shown for larvae > 300 μm shell length approaching metamorphosis. Like adult oysters, the larvae regulate the amount of food ingested in different concentrations by adapting their filtering activity to different concentrations. When fed mixed algal cultures, food uptake is greater than by feeding monocultures.