Growth, density, and body size of the deposit-feeding amphipod Hyalella azteca, and its food, epibenthic algae, and sediment microflora, were greatest in shallow-water areas of Marion Lake. The vertical distribution of Hyalella was limited to the upper 2 cm of sediment cores. Highest concentrations of sedimentary chlorophyll and lowest concentrations of nondigestible ligninlike material also occurred at the sediment surface.In laboratory substrate-choice experiments, Hyalella differentiated between sediments with different concentrations of microorganisms, and growth depended upon the quantity of microflora in the diet. In Marion Lake, increased growth of Hyalella during June was independent of temperature and closely correlated with increased rates of epibenthic primary production.Egg production, related to body size in a nonlinear manner, began during May as growth rates increased. As a combined result of egg production and juvenile survival, the maximum density of Hyalella in Marion Lake was reached in August.