Multispecies stream microcosms were used to test the toxicity of the pyrethroid fenvalerate (FV) associated with suspended particles in order to simulate a typical runoff exposure scenario. Stream microcosms were exposed for 1 h in triplicate to 0.0, 13.6, 136, or 1,365 μg/kg FV and effects were monitored for 93 d. Experimental design allowed for detection of interspecific effects on the emergence and thus survival of the caddisfly species Limnephilus lunatus Curtis and of intraspecific effects on the spatial distribution of adult and juvenile Gammarus pulex L. (Amphipoda). Exposure at 136- and 1,365 μg/kg resulted in a significant acute increase in drift. Survival of L. lunatus was significantly reduced in the 1,365-μg/kg treatment during single-species exposures. When other species were present, survival of L. lunatus was significantly reduced at 136 μg/kg. A similar increase in test system susceptibility was observed in relation to the spatial distribution of G. pulex. Juvenile individuals avoided areas with high numbers of adult amphipods, which may prey on the juveniles. This avoidance was significant in the control and the 13.6-μg/kg treatment but did not occur at higher levels of exposure. This study highlights the ecotoxicological importance of field-relevant levels of particle-associated hydrophobic chemicals that transiently increase during runoff events. Interspecific and intraspecific interactions can alter the test results; i.e., significant lethal and sublethal effects are measurable at FV levels approximately an order of magnitude lower than when biological interactions are excluded.