Two 10-month exposures of malathion and butoxyethanol ester of 2,4-D under continuous-flow conditions were made to determine the effect of these pesticides on reproduction and growth of fathead minnows, Pimephales promelas Rafinesque. Results show that 1/45 and 1/19 of the 96-hour median tolerance limit (TLm) concentrations of malathion and butoxyethanol ester, respectively, will not harm growth and reproduction of fathead minnows during continuous exposure. We suggest that these two fractions applied to TLmconcentrations determined for other species and other types of water should estimate accurately safe concentrations for other fishes to these pesticides.