Food habits, net-spinning activity, and whole-animal mercury concentrations in Hydropsyche morosa Hagan were examined monthly over a one year period on the South River, Virginia. Gut content analysis revealed seasonal patterns in the consumption of food that was correlated with net-spinning activity. Between April and October, when feeding nets were widespread, detritus represented between 72 and 94% of the total volume of food material found in the guts, while algae represented less than 18%. However, between November and March, when feeding nets were rare or absent, the relative contribution of each of the three food types (i.e., detritus, algae, and animal) changed significantly. Detritus declined to between 51 and 60%, and algae increased to between 39 and 47% of the total food volume. Whole-animal mercury concentrations ranged from 0.14 to 1.20 µg g⁻¹ and were significantly higher in the summer. Seasonal differences were not related to environmental mercury levels and only weakly related to body size. In contrast, whole-animal mercury levels were strongly related to seasonal changes in diet. In summer, when H. morosa larvae were filtering highly contaminated seston, whole-animal mercury levels were high. Conversely, in winter, when larvae were grazing less contaminated algae, whole-animal mercury levels were significantly lower. The primary component of seston was detritus (>81%) throughout the year. Therefore, the high mercury concentration in seston was probably associated with detritus. As a result, a significant relationship was observed between whole-animal mercury concentration and the relative amount of detritus consumed. These patterns indicate that in streams with highly productive hydropsychid populations, the rate of processing of mercury and other heavy metals may be related to seasonal changes in feeding behavior.