AbstractChlorinated phenols, which are used primarily as wood preservatives and fungicides, are present in most air, water, and soil samples in industrialized areas as well as in the urine of most people. We have examined the ability of phenol and the 19 isomers of chlorophenol to induce DNA damage using the Microscreen prophage‐induction assay in Escherichia coli. Seven of the isomers (2,3,4,‐tr, 2,4,5‐tri, 3,4,5‐tri, 2,3,4,5‐tetra, 2,3,6‐tri, 2,4,6‐tri, and pentachlorophenol) induced prophage lambda in the presence of S9, with the first three being ∼ 10 times more potent than the last three. The more potent isomers have either one or no chlorine atom ortho to the OH group; whereas the less potent isomers have two chlorine atoms ortho to the OH group. Although none of the 20 compounds is mutagenic in Salmonella, the prophage‐induction results agree with findings by others that most of these seven isomers are clastogenic, are associated with cancer and chromosomal aberrations in humans (pentachlorophenol), and are carcinogenic in radents (2,4,6‐tri and pentachlorophenol). A likely basis for the genotoxicity of the seven isomers involves the metabolism of the parent isomer to a chlorohydroquinone, which can form a chlorobenzosemiquinone in the presence of oxygen. These two metabolites can produce free radicals that can cause DNA strand breaks, resulting in prophage induction in E. coli or, possibly, the chromosomal aberrations/cancer associated with human exposure to chlorophenols.