Genetic toxicants are present in polluted marine environments and may represent a long-term threat to populations of marine organisms. A cytogenetic model is useful to study the effects of these toxicants. The polychaete, Neanthes arenaceodentata, was chosen as such a model because it has a suitable karyotype, is easily cultured, and represents an ecologically important group of organisms. This paper presents details of an in vivo application of sister chromatid exchange (SCE) analysis, a sensitive cytogenetic technique, to this marine worm. In earlier studies, N. arenaceodentata exhibited a dose response to mitomycin C (MMC) at concentrations comparable to those that elicited responses in in vivo mammalian systems. Exposure to 5 × 10 −7 MMMC for 48 h increased the frequency of SCE in the worm from a baseline value of 0.14 exchanges/chromosome to 0.5 exchanges/chromosome. Positive SCE responses in the worm have also been demonstrated in this study for other known, direct-acting mutagens such as 5-bromodeoxyuridine and methylmethanesulfonate, as well as for compounds that need metabolic activation luch as benzo[ a]pyrene, dimethylnitrosamine and cyclophosphamide. These results imply that N. arenaceodentata can metabolize promutagens and suggest that the worm may be sensitive to a broad spectrum of genetic toxicants. The significance of these findings, as well as directions for future research, are discussed.