A major underlying motivation for seriously studying carcinogenesis in aquatic animals is the concept of utilizing selected lower animal species as models in understanding neoplasia and the neoplastic process. Numerous examples may be cited which illustrate the contribution that ectothermic animals, as models, have made to the principles of pathology, physiology, biomedicine, and now, perhaps, oncology. The purpose of the paper is to provide an overview of the present status of bivalve molluscs and teleost fishes in environmental and experimental studies of carcinogens and cancer.