Selected species of stoneflies, caddisflies, mayflies, dipterans, amphipods, and snails were exposed, in the laboratory, to fenvalerate or permethrin in a flowing-water test system for up to 28 days. Both behavior and death were monitored during the exposure. The LC 50 values decrease with increasing exposure time. Behavior changes or death were seen at concentrations as low as 0.022 μg/liter for fenvalerate and 0.030 μg/liter for permethrin. In the fenvalerate exposures, amphipods were the most sensitive, followed by the insects and snails. In the permethrin exposures, changes in normal behavior occurred within hours, but death did not exceed 50% until at least 14 days of exposure. Pesticide accumulation was determined in fenvalerate-exposed snails and permethrin-exposed stoneflies. The range of accumulated fenvalerate in snails was 177 to 1,286 times greater than water concentration. The stoneflies' concentration factor for permethrin ranged from 43 to 570.