The lethal and mutagenic effects of p-methyl-, m-chloro-, p-chloro- and unsubstituted styrene oxide on Salmonella typhimurium (TA 100) were investigated. At equal concentrations, p-chlorostyrene oxide was more lethal than p-methyl-, m-chloro- or unsubstituted styrene oxide. When the survival fraction was 0.8 or more, the mutagenicities of these compounds increased in the order: m-chlorostyrene oxide = p-chlorostyrene oxide < styrene oxide < p-methyl-styrene oxide. The mutagenicities of these compounds depended only on the reactivity of their benzylic site; the reactivity at their primary site and their partition coefficients appeared to have no effect.