These three stories exemplify the uniqueness of fish models in their abilities to answer important biological questions. The first one identifies the major UV-induced damage (pyrimidine dimers) that is responsible for tumor induction. Thyroid cells from isogenic fish ( Poecilia formosa) were exposed to UV in vitro, then either exposed or not to photoreactivating light that monomerizes dimers, and the cells were injected into isogenic recipients. In the absence of photoreactivating light, the recipients developed tumors; in its presence, there were very few tumors. The second story describes our use of backcross hybrids of Xiphophorus as a model for melanoma induction by several UV and visible wavelengths. All the wavelengths were effective. (Squamous cell carcinomas in mice are induced preferentially by wavelengths <320 nm.) The data strongly suggest that light absorbed by the black pigment melanin damages DNA. The third story is designed to determine the mutagenic effects on sperm of the high atomic number, high-energy (HZE) nuclei present in cosmic rays by measuring mutations in progeny of exposed male medaka.