Behavioral time budgets were constructed for larval black flies of the Simulium jenningsi and S. tuberosum groups in a recirculating flume. Larvae spent the majority (54–80%) of time filter feeding. Flick rates of the labral fans, which averaged 0.7–1.9 flicks/sec, did not differ significantly between regimens of 0 mg/l and 10 mg/l of food. Larvae consistently flicked the fan nearest the water surface significantly more often than the lower fan. They also tended to flick the right fan more than the left, although this trend was significant in only one of four treatments. Although handedness cannot be dismissed, the differential flick rate appeared to be related to the position of the fans relative to the water surface, possibly because of different particulate concentrations in the microcurrents received by the upper and lower fans. Cessation of feeding by larvae of the S. jenningsi group from rivers regularly treated with Bacillus thuringiensis var. israelensis was independent of exposure to this biological control agent. Of larvae that ceased to feed, the mean time to cessation of feeding after exposure to Bti was 12–18 s, sufficient time to ingest a lethal dose.