Rates of water processing and particle retention were determined in veligers and post-metamorphic juvenile M. edulis, reared in the laboratory, by measuring the rate at which particles of different sizes were cleared from suspensions. Veligers cleared 2.5-3.5 μm particles at the highest rate, and particles smaller than 1 μm or larger than 8-9 μm could not be consumed. The veligers cleared the water at a constant rate for 8 hours (11.4μl • h-1 • larva-1) at concentrations of 1.5-5.5 × 103Iso- chrysis galbana cells ml-1. At abnormally high algal concentrations the clearance fell significantly, probably because the capacity of the stomach was exceeded, while the grazing rate became inde pendent of the algal concentration (80 cells • h-1 • larva-1). The retention efficiency for young post- metamorphic mussels (1-4 mm shell length) was 100 % for particles down to 4 μm, decreasing to 20 % for 1 μm particles. The filtration rate (F, ml • h-1) in the juveniles (total organic dry weight, W, 70-10000 μg) could be expressed as F = 0.025 W1-03, and the oxygen consumption rate (R, nl • h-1) in the same animals as R = 0.287 W1.14. The water processing potential, i.e. the rate of water processing in relation to the overall energy requirement, is shown to be of the same order of magnitude throughout the ontogenetic development, 15-50 I water being processed per ml of oxygen consumed.