In order to know whether polychaetes could be used as an appropriate organism for the detection of genotoxicity, DNA strand breaks were evaluated in blood cells of a nereidae worm (Perinereis aibuhitensis) exposed to various aquatic chemical pollutants (e.g. Cd, Pb, Pyrene, Benzo[a]pyrene). Hydrogen peroxide increased DNA strand breaks up to the highest concentration (10 μM). Higher concentration than 0.1 μM showed a significantly more DNA damage than control. Cadmium and lead also showed higher DNA damage than control, over 1.0 and 1 ㎍/L, respectively. In case of pyrene, DNA damage was detected even at 0.001 ㎍/L. However, DNA damage decreased due to apoptosis at the highest concentration of pyrene and Pb. This study suggested that the polychaetous blood cells could be used effectively for screening genotoxic contaminants in the environment.