A respirometer was utilized to determine the energy cost of food utilization in the bluegill Lepomis macrochirus. Prior to the study, fish were acclimated to the experimental photoperiod (16 hr light) and temperature (15, 20 and 25 C) for 90 and 14 days, respectively. Each fish was tested in the respirometer for 3-day periods. After an initial acclimation period of 24 hr, metabolic rates were determined at 2-hr intervals for the next 48 hr and were converted to their caloric equivalents. Fish were fed to satiation on mayfly nymphs (Hexagenia limbata) in the morning and evening of day 3. We calculated the energy cost of food utilization by determining the increase in energy of respiration subsequent to feeding.