Fathead minnows (Pimephales promelas) were exposed to technical grade propanil (3',4'-dichloropropionanilide) in a flow-through diluter system to determine acute lethality. LC50 values were 11.5, 10.2, 8.6, and 3.4 mg.per L at 24, 48, 96, and 192 hr, respectively. Eggs, newly hatched fry, and juvenile fish of this species were similarly exposed but at lower concentrations and for a period of 58 days. The 58-day 'no-effect' concentration was between 0.4 and 0.6 micrograms.per L, based upon the physiological parameters of length and dry weight of juvenile fish. 14C-Propanil did not bioconcentrate significantly in fathead minnows (1.6x for parent propanil in whole body). Rainbow trout (Salmo gairdneri) readily metabolized propanil, forming at least ten products. One metabolite recovered from trout bile was identified as either 3',4'-dichloro-2-hydroxypropionanilide. The technical grade propanil also contained 0.67 mg.per g of 3,3',4,4'-tetrachloroazo-benzene as a contaminant.