Whole fish were collected at 52 sites during 1995-99 to evaluate the occurrence and distribution of selected organochlorine compounds in the Mississippi Embayment Study Unit. Samples were collected as part of the U.S. Geological Survey National Water Quality Assessment Program. From 5 to 8 fish were collected at each site; the fish were composited, and an aliquot of the tissue was analyzed for 28 organo-chlorine compounds, which included pesticides, pesticide degradates, and polychlorinated biphenyls. The use of these organochlorine compounds has been discontinued or severely restricted within the United States, but the continued detection of these compounds or their degradates in the air, water, soil, and biota in national surveys, coupled with known environmental problems associated with these compounds (such as a long half-life and the propensity to accumulate in living tissue), is cause for continued interest in their environmental fate. At least one organochlorine compound was detected in every fish-tissue sample, and as many as 15 different compounds were detected in some. The most frequently detected com-pounds were the degradates of p,p'-dichlorodiphenyltrichlo-roethane (p,p'-DDT); p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE) was detected in every sample above the method reporting limit, and p,p'-dichlorodiphenyldichloroethane (p,p'-DDD), was detected in 94 percent of the samples. Polychlorinated biphenyl compounds and dieldrin were detected in 83 and 78 percent of fish-tissue samples, respectively. Because these were whole fish samples, the results are not directly comparable to human health standards, which are based on fish fillets. Comparison of these results, however, to the guidelines for the protection of fish-eating wildlife indicates that concentrations of the p,p'-DDT degradates and toxaphene continue to be of environmental concern.