In this field study of the effects of organic contaminants in San Francisco Bay on starry flounder Platichthys stellatus, we have focused on indicators of contaminant exposure and their relationship to changes in reproductive physiology and biochemistry. We also measured micronucleus occurrence as an indication of the exposure of these fish to environmental mutagens. Two large collecting efforts were made—one during the middle part of the reproductive cycle (November-early December 1986) and one at the time of spawning (February-March 1987). For the former period P. stellatus were collected from five localities (four in San Francisco Bay and one near the mouth of the Russian River). A variety of measures were made on these fish that might indicate the nature and extent of contaminant exposure, the alteration of normal reproductive processes, and genetic effects: (1) trace organic contaminants in the liver, mainly chlorinated hydrocarbons, (2) constituents and in vitro enzymatic activities of the hepatic P-450 microsomal system, (3) oocyte development in maturing females, (4) plasma concentrations of vitellogenin and titers of sex steroids, and (5) the incidence of micronuclei in circulating erythrocytes.