All classes of vertebrates have similar inducible liver microsomal monooxygenase systems that can convert certain chemicals to proximate carcinogens. Chemical carcinogenesis has been confirmed experimentally in over a dozen species of fish and amphibians with various nitrosamines, polynuclear aromatics and aflatoxins. Therefore, neoplasms prevalent in populations of hagfish, tomcod, English sole, bullhead, walleye, sauger, tiger salamander and other species from specific polluted habitats are suspected of having a chemical etiology.