High abundance of filter-feeding lotic insects near lake outlets has been attributed to the high quality/quantity of seston available in this habitat. Accordingly, a decrease in food quality or quantity with increased distance from a lake outlet might be detectable as a decrease in insect size. We examined changes in larval size of the black flySimulium truncatumwithin the first 120 m of a stream draining a small lake from the Avalon Peninsula, Newfoundland. The observed decrease in larval size with increased distance from the outlet was consistent with a decreasing food hypothesis. Laboratory experiments also showed that larval size was dependent on food supply.