The relationships among concentration of sedimentary organic matter, deposit-feeding rate, and growth rate of Abarenicola pacifica were measured in laboratory experiments. When natural sediments were diluted with inorganic filler, feeding rate increased as the concentration of enzymatically available protein in the sediments decreased from ambient levels. Maximum feeding rates (2.4–7.0 g dry sediment · worm −1 · day −1) occurred at ≈ 0.05–0.1 mg protein · (g dry sediment) −1. Further decreases in sedimentprotein concentration resulted in decreased feeding rates. This type of functional response curve is similar to that reported for suspension-feeders and fits predictions of feeding models based on the energymaximization assumption. These models predict that peak feeding rates occur at the food concentration where the animal's energy-uptake mechanisms are saturated. We found, however, that growth rate decreased steadily as sediment-protein concentration decreased. Maximum net rate of energy gain did not correspond to the peak in sediment-processing rate.