Fundulus grandis, the gulf killifish, was provided a prey choice of grass shrimp, Palaemonetes pugio, and juvenile sheepshead minnows, Cyprinodon variegatus. Fundulus grandis consumed a greater proportion of grass shrimp when both prey species were exposed to the pesticide methyl parathion than when prey were not exposed. The relative number of shrimp eaten increased with the concentration of methyl parathion. Such an effect in an estuary could result in altered species composition and species diversity.