Flow-through seawater bioassays of 28-days duration were conducted with the organochlorine pesticides heptachlor and toxaphene to determine their toxicity to and bioconcentration by embryos and fry of the sheepshead minnow (Cyprindon variegatus). At technical heptachlor measured concentrations of 4.3, 3.5, 2.2, 2.0, and 1.2 micrograms/l (ppb), test animal survival was 1, 5, 61, 79, and 88%, respectively. At toxaphene measured concentrations of 2.5, 1.1, 0.6, 0.3, and 0.2 micrograms/l, test animal survival was 10, 85, 79, 88, and 80% respectively. Average standard length of fry continuously exposed from fertilization to heptachlor concentrations of 4.3 and 3.5 micrograms/l was significantly reduced. Concentration factors (concentration in fish/measured concentration in water) for heptachlor averaged 3,600 and for transchlordane averaged 8,600. Heptachlor epoxide and cis-chordane were also present in the fish. Concentration factors for toxaphene in fry averaged 9,800. Various histopathological characteristics not seen in control fish were observed in the liver, kidney, pancreas, and intestine of the few fish that survived 4.3 and 3.5 micrograms/l of heptachlor.