Hemlock Lake, a eutrophic and meromictic Michigan lake, was artificially aerated by hypolimnetic aeration. This aeration system involved a special aeration tower and compressed air injection. Hypolimnetic oxygen concentrations increased from zero to saturation during aeration. Thermal stratification was gradually reduced by the aeration, but the lake remained thermally stratified for 10 weeks during aeration. Before aeration, rainbow trout (Salmo gairdneri Richardson) were limited to shallow depths by the anaerobic hypolimnion. They distributed to the deepest depths soon after aeration began and occupied the entire lake during artificial aeration.