Availability, economy, latency of tumorigenic response, and ease of maintenance and exposure are commonly cited advantages of small fish species in carcinogenesis bioassays. Furthermore, carcinogen metabolism and mechanisms of carcinogenesis in small fish species appear similar to those processes in large fish as well as in rodents. Practically, small fish can be used to test waterborne carcinogens having a variety of expected mechanisms. Direct-acting genotoxic carcinogens are tested in brief static exposures. Seven species exposed to methylazoxymethanol acetate (MAM-Ac) under identical conditions demonstrated species dependent neoplasms latencies, dose responses, and types of neoplasms induced. MAM-Ac exposures further revealed two new tumor models, retinal medulloepitheliomas in Japanese medaka and exocrine pancreatic neoplasms in the guppy. The retinal neoplasms involved elements of both sensory retina and pigment epithelium and their analysis should lead to a better understanding of several retinal neoplasms in other organisms. The pancreatic neoplasms involved mainly acinar cell neoplasms. Most indirect acting genotoxic carcinogens require longer exposures than direct-acting ones. Polynuclear aromatic hydrocarbons such as benzo(a)pyrene (BaP) and 7,12-dimethylbenzanthracene (DMBA) induce tumors in medaka and guppy following waterborne exposures. BaP induced liver tumors in both species whereas DMBA induced several types of extrahepatic tumors as well as hepatic ones. Preliminary studies also indicate that some halogenated organic compounds cause hepatic neoplasms in exposed medaka and guppy. This study was supported by Public Health Service contract N01CP­26008/61070 from The National Cancer Institute, the U.S. Army Medical Research and Development Command, and by the American Petroleum Institute.