An entire life-cycle toxicity test with the estuarine sheepshead minnow (Cyprinodon variegatus) and the organochlorine insecticide Kepone in seawater showed that the maximum acceptable toxicant concentration (MATC) lies between 0.074 and 0.12 micrograms Kepone/l, based on reduced standard length of progeny at 0.12 micrograms/l and higher concentrations. The application factor range (MATC/96-hour LC50) for sheepshead minnows exposed continuously to Kepone is from 0.001 to 0.002. Other adverse effects of Kepone exposure were: external signs of poisoning in fish exposed to concentrations greater than or equal to 0.074 micrograms/l, fatty degeneration of the liver and histological evidence of vertebral damage in fish exposed to 0.78 micrograms/l, reduced growth of parental fish at 0.39 and 0.78 micrograms/l, reduced fecundity and fertility of eggs at 0.78 micrograms/l, and reduced survival of embryos produced by fish exposed to 0.78 micrograms/l. Average bioconcentration factors (concentration in tissue averaged measured concentration in water) were: 21-day juveniles-2,600; 42-day juveniles-2,800, adult males-7,600, adult females-5,700, eggs-3,300, and 28-day juvenile progeny-3,100.