This study investigated the antigenotoxic effects of Synurus deltoides extract on the mutagenesis induced by benzo[a]pyene(B[a]P). About 80% and 90% antimutagenic effects were observed in the presence of over 200$mu extrm{g}$/plate of methanol extract of Synurus deltoides against Salmonella typhimurium TA98 and TA100 induced by B[a]P, respectively. The methanol extract itself did not induce an increased frequency of micronucleated polychromatic erythrocytes(MNPCE) irrespective of the sampling time(up to 72h), while the treatment with benzo[a]pyrene(B[a]P) at 150mg/kg significantly increased (p<0.05) the incidence of MNPCE. The strongest relative frequency of MNPCE was observed at 36h after injection of B[a]P and the most significant reduction (p<0.05) in the frequencies of MNPCE was occurred at the feeding of the methanol extract to mice 12 h before injection of B[a]P. The most significant reduction (p<0.05) with 48% was observed in the frequencies of MNPCE when 50 mg/kg of the methanol extract was given to the mice 12h before injection of B[a]P, while the strongest relative frequency inhibition was 54% at the multiple feeding of 5mg/kg of the methanol extract e time every day for 5 days on th frequencies of MNPCE induced by 150 mg/kg of B[a]P. These results indicate that the methanol extract of Synurus deltoides have a strong modulatory effect on benzo[a]pyrene induced MNPCE.