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 polychaeta, 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 times 10 to the 7th power M MMC 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 such 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.