Stoneflies, caddis-flies, isopods, snails and bullheads were exposed to endrin or methoxychlor in a flowing-water test system for 28 days. Endrin was more toxic than methoxychlor. Behavior changes in the caddis-fly Brachycentrus americanus and in the stonefly Pteronarcys dorsata were observed within 4 days at 0.07 and 0.15 microgram/liter of endrin, respectively. The LC50 values decrease with increasing exposure times. The 28-day LC50 was less than 0.03 microgram/liter for B. americanus and 0.07 microgram/liter for P. dorsata. The 28-day LC50 for the bullhead Ictalurus melas was 0.10 microgram/liter. Methoxychlor, at the concentrations tested, did not affect all the species. The isopod Asellus communis was the most sensitive; behavioral changes occurred within the first 4 days. The 28-day LC50 was 0.42 microgram/liter. The LC50 for the caddis-fly Hydropsyche sp. decreased from 2.9 micrograms/liter at 14 days to 1.3 microgram/liter at 28 days. The stonefly P. dorsata and the snail Physa integra did not die at the highest concentration tested (4.2 micrograms/liter), although behavioral changes were seen at 2.2 micrograms/liter for the stonefly.