In this study, a total of 180 fish specimens (wels: Silurus glanis-60; common carp: Cyprinus carpio-60; bleak: Albumus escherichii-60) of ages between one and two were caught at three different stations in Sariyar Reservoir. The histological changes in the livers and kidneys of three different species of fish were detected microscopically and evaluated with quantitative analyses. Also, organochlorine pesticide residues (OCP) have also been determined in the water and sediment samples and in the adipose tissues of fish caught in these stations. Results show that the reservoir was polluted by different kinds of OCP compounds and these chemicals have accumulated in the fish tissues. As a result of these analyses, histopathological changes were observed in the livers and kidneys of fish specimens, such as mononuclear cell infiltration, congestion and nuclear picnosis. Also intra-cytoplasmic cholestasis in their livers and tubular degenerations in the kidneys were observed. The incidences of the histopathological changes in wels and carps were found to be higher than bleak. Furthermore, histopathological changes in fish samples caught from Usakbuku were much more than the samples caught from other stations (Sariyar and Nallihan Bird Paradise Stations). In this study the possible reasons of histopathological changes were evaluated with respect to different fish species and localities and also the findings were evaluated in relation to OCP contamination. (c) 2006 Elsevier B.V. All rights reserved.