The mutagenic activities of several phthalate esters have been evaluated in an 8-azaguanine resistance assay in Salmonella typhimurium. Three phthalate esters were found to be mutagenic: dimethyl phthalate, diethyl phthalate and di-n-butyl phthalate. A number of other phthalate esters were not found to be mutagenic, including di(2-ethylhexyl) phthalate, di-n-octyl phthalate, diallyl phthalate, diisobutyl phthalate and diisodecyl phthalate. A metabolite of di(2-ethylhexyl) phthalate, 2-ethylhexanol, was also noted to be mutagenic. The mutagenic activity of this agent and others in this series was dose dependent but weak. No dose-response curve exceeded more than 3.5 times background at maximally testable concentrations. A liquid suspension histidine reversion assay of dimethyl phthalate showed levels of mutagenic activity similar to that observed in the azaguanine resistance assay. The data suggest a need for further investigation of the mutagenic potential of these agents in other assay systems.