Abstract.-The growth of young-of-the-year bluegills Lepomis macrochirus was measured in six experimental ponds and simulated with an individual-based model. In the ponds the young bluegills grew at a rate of about 0.6 mm/d for 3-4 weeks. An abrupt reduction in growth rate to about 0.2 mm/d occurred when total zooplankton density (exclusive of rotifers) decreased below about 50 organisms/L, and growth rate decreased to nearly zero by September. The model included daily foraging for several sizes of open-water or benthic prey and a revised set of bioenergetics parameters for bluegill. The simulations suggest that the initially rapid growth rate was near the limit set by maximum daily ration; the fish may have obtained full rations even with suboptimal foraging during this phase. Over a wide range of fry densities, the time of growth reduction and the average final fish size at the end of the growing season were strongly density dependent, both in the simulations and the ponds. Two natal cohorts started 10 d apart in the simulations. The size-frequency distributions produced by this individual-based model showed that these two cohorts remained distinct at starting densities below about 1 fish/m³, but tended to overlap in size at higher densities.