Abstract: Hourly measurements, for periods of 24 to 33 h, were made in the laboratory of filtration rate and cell clearance rate of 39 individual Argopecten irradians concentricus (Say) and Crassostrea virginica (Gmclin). Bivalves red on suspensions of algae (Dunaliella tertiolecta Butcher, Isochrysis galbana Parke, or Thalassiosira pseudonana (Hustedt)) whose concentration was maintained at a nearly constant level throughout each experiment. Neither local tidal sequence nor laboratory day: night cycles exerted a significant influence on scallop or oyster filtration behavior. In Argopecten irradians filtration activity either remained relatively constant throughout the experimental period or stabilized at a constant level after an initial period of steady decline. There was an inverse relationship between suspended algal concentration (0.94-9.66 mg · 11) and filtration rate of A. irradians, so that the average amount of algae cleared hourly was similar throughout this range of concentrations. Mean filtration rate for all experiments with scallops was 4.71 · h 1) g dry wt-1, but averaged 5.7 1 · h-1 g dry wt-1 when amb1ient concentration was<1.5 mg · 1-1. Filtration behavior of Crassostrea virginica was generally characterized by alternating periods of high and low activity. Peaks of oyster filtration activity occurred two to three times per day. and the period between peaks did not vary with experimental algal concentration (1.7-6.7 mg · 1-1). Oysters filtered actively for 80°0 of all hourly periods in suspensions of Thalassiosira pseudonana and 91°0 in suspensions of Isochrysis galbana, mean filtration rate for Crassostrea virginica was 1.51 · h-1 · g dry wt-1for all measurements, and 1.91 · h1 · g dry wt-1 during hourly periods of active filtration. These results indicate that scallops can collect food continuously, and, in the range of concentrations of suspended matter typical of coastal environments, can respond to environmental variations quickly enough to collect a relatively constant supply of food over time. In similar concentrations, filtration of oysters is much more variable. Fluctuations in filtration of oysters in the laboratory could not be related to tidal or diurnal cycles or to food availvbility. Although their frequency does suggest a tidal component in filtration, the most probable explanation for variations in cell clearance rate is that they serve to regulate food levels in the stomach to permit a relatively constant level of intracellular digestion.