The antimutagenic activity of a methanol extract of Ecklonia stolonifera (Laminariaceae) against aflatoxin $B_1;(AFB_1)$ was demonstrated with the Salmonella typhimurium assay. The numbers of revertants per plate decreased significantly when this extract was added to the assay system using S. Salmonella typhimurium TA100. The methanol extract also exhibited significant inhibitory effects on the micronuclei formation in mouse peripheral blood reticulocytes and the DNA damage in mouse spleen lymphocytes induced by N-methyl-N-nitrosourea (MMU) and benzo(a)pyrene (B(a)P). The MeOH extract was then sequentially partitioned with $CH_2Cl_2,;CH_2Cl_2$ insoluble intermediate, EtOAc, n-BuOH, and $H_2O$. All fractions possessed antimutagenic activity but the $H_2O$ fraction was inactive. Among active fractions, the EtOAc and $CH_2Cl_2$ insoluble intermediate fractions showed the highest activity. Column chromatography using $SiO_2$ and Sephadex LH-20 yielded phloroglucinol from the EtOAc fraction. Phloroglucinol also demonstrated significant antimutagenic activity, and inhibitory effects on the micronuclei formation in mouse peripheral blood reticulocytes and DNA damage in mouse spleen lymphocytes induced by MMU and B(a)P.