The marine environment receives a wide variety of chemical inputs, many of which have the potential to damage DNA or interfere with the process of cell division. Here we describe a new assay based on the early embryo and larval stages of a planktonic spawning, tube dwelling marine worm, Pomatoceros lamarckii, which for experimental purposes has the advantage of producing large numbers of ripe gametes throughout the majority of the year. One of the most promising end-points is the use of dividing cells to detect anaphase aberrations such as lagging chromosomes, tripolar anaphases, acentric fragments and chromosome bridges. Apart from the reference mutagens mitomycin C and cyclophosphamide and the well-documented spindle poison colchicine, we tested the fungicide carbendazim, a primary metabolite of the fungicide benomyl, and thiabendazole, a pesticide and antihelminthic drug; both of which are known to act as aneugens in other test systems. In addition we tested sodium hypochlorite, a widely used oxidizing agent and disinfectant, di-butylphthalate, a commercial plasticizer and suspected aneugen, and sodium chloride, a recognized non-genotoxin. Significant increases in the frequency of anaphase abnormalities occurred with most test compounds at relatively low concentrations, confirming the sensitivity of the new assay. Sodium chloride yielded a negative response except at the highest non-relevant concentrations, where some chromatid stickiness was observed. In addition, the developmental consequences of exposure to these compounds were assessed in 4-8 cell embryos and at 48 h once the embryos had metamorphosed into free swimming larvae. Mitotic inhibition and anaphase aberrations were found to be a more sensitive indicator of genotoxic exposure than larval development, although there was a suggestion of a possible mechanistic link between aneugenicity/clastogenicity and larval fitness. The new test assay provides a rapid and inexpensive method for screening chemicals and effluents destined for release into the marine environment for potential gamete effects.