DNA single strand breaks in the liver of Boleophthalmus pectinirostris with benzo ( a) pyrene exposure were detected using DNA alkaline unwinding assay under experimental ecological conditions. It turned out that DNA single strand breaks were effected by exposure time and exposure concentration and were time-dependent and dose-related. The repair of DNA damage was weak because no significant change was observed after the f ishes were transferred to clean sea water. All of the results show that DNA single strand breaks are suitable to be the bioindicator of DNA damage with BaP exposure.