In this study, 80 carp specimens (Cyprinus carpio) (16 fish from each station) of ages between 1-2 were caught at five stations in the upper Sakarya river basin the four seasons between October 1995 and October 1996. The structural changes of gill, liver and spleen tissue of the carp were examined. The gills of fishes which live in aquatic habitat exhibit tissue lesions caused by water pollutants, because they are in direct contact with their environment. In the results of this study, it was found at secondary lamellae fused together, and the gills are the most affected organs. Other histopathological lesions, hyperplasia, hypersecretion of mucous cells and separation of the primary lamella were found in the gill epithelium. Also, hypertrophy, congestion and mononuclear cell infiltration were detected in the liver tissue especially in the peripheral region of lobules. In the kidney tissue, pathological lesions which included hypertrophy, nuclear pyknosis and disruption of the absorption surface, were detected especially in tubular epithelial cells. Proximal tubular degeneration was observed in the kidney tissue.