Contaminants entering coastal marine environments may affect the genetic constitution of exposed organisms by causing shifts in gene pool composition through selective pressures or by acting directly on the genetic material to cause damage. The latter problem is referred to as genotoxicity. The article reviews genotoxicity studies done with marine polychaetes, with special emphasis on the chromosomal response of sister chromatid exchange (SCE). Chromosomal responses have been studied in the polychaete Neanthes arenaceodentata exposed to pure compounds and radiation, and in the polychaete Nephtys incisa exposed to a complex mixture. All of the published results to date have come from two laboratories, the U.S. National Laboratory at Livermore, California (Harrison and co-workers) and a U.S. EPA Laboratory at Narragansett, RI (Pesch and co-workers). The article reviews and summarizes the work in the context of assessing risks associated with mutagens and carcinogens in marine environments.