Abstract Chemical contaminants may play a role in the etiology of external skin lesions on walleyes Stizostedion vitreum in the Great Lakes. We examined the population of walleyes spawning in the Thames River, Ontario, for skin lesions, and compared contaminant levels in walleyes with conspicuous lesions to contaminant levels in visibly normal walleyes. In 1987, 9% of postspawning downstream migrants exhibited skin lesions. Twenty affected fish, many with multiple abnormalities, were autopsied and the external skin lesions were examined histologically. Eight walleyes were affected by only lymphocystis, seven by only dermal sarcoma, four by both conditions, and one by a calcareous nodule; 68% of the individual lesions affecting these walleyes were lymphocystis. Muscle tissue from the walleyes with lymphocystis or dermal sarcoma had lower concentrations of polychlorinated biphenyls, hexachlorobenzene, and the DDT metabolite p,p-DDE than did muscle from unaffected walleyes. Thus, it appears unlikely that con...