Water flow speeds and insecticide contamination may change predator-prey interactions between the California killifish and a spionid worm. We performed two experiments in a laboratory flume at two flow speeds (6 and 15 cm/s). We quantified changes in the worm feeding behaviors when fish were actively feeding on the worms, caged upstream, or absent from the flume. In the presence of actively feeding fish, worms reduced the time they spent feeding by over 50% relative to when fish were absent. Fish caged upstream of worms had no effect on their feeding activity, and effects due to fish presence did not differ between flow speeds. We also tested the effects of foraging fish on the feeding behaviors of worms that had undergone a 96-h exposure to 3-ppb chlorpyrifos (CPF). Worms exposed to CPF reduced their feeding time by ~25% relative to unexposed worms, but this effect only occurred in the absence of predatory fish and was not influenced by flow speed. We also conducted two laboratory flume experiments to quantify whether predator-prey interactions varied with sub-lethal exposure of fish to CPF at four flow speeds (6, 9, 12, or 15 cm/s). The flume had a prey patch (98 cm2) centered in a larger sediment area (300 cm2). Fish were exposed to one of three CPF treatments (1-ppb CPF, 3-ppb CPF, or an acetone control) 96 h prior to each flume run. Killifish not exposed to CPF directed ~70% of their bites at the prey patch at 6 cm/s, and this decreased as flow increased. At speeds of 15 and 18 cm/s, the location of bites was indistinguishable from a null model of random biting. Killifish exposed to CPF directed ~ 40% of their bites at the prey patch at a flow of 6 cm/s, but at 12 and 15 cm/s there was no patch selection. There was a ~30% reduction in CPF-exposed worms feeding activity at flow speeds greater than 6 cm/s. Neither fish nor worms altered their feeding behavior when the other was exposed to CPF. This study highlights how environmental conditions can alter how anthropogenic toxicants affect predator-prey interactions.