Micronucleus assays with fish have been shown to be useful in vivo techniques for genotoxicity testing, and show potential for in situ monitoring of water quality. In this paper, we review the literature on the clastogenic effects of chemical and physical agents on fish cells, with emphasis on the induction of micronuclei in teleosts. Included in the review is a description of the mechanisms for formation of micronuclei in cells, and a summary of the various techniques that have been used for micronucleus analysis in fish. This review is directed to assisting laboratories in the development of fish genotoxicity assays for water quality monitoring.