Four nitroarenes were tested in two standard genotoxicity assay systems using three well-known bacterial tester strains. The results were as follows: 4-nitroquinoline l-oxide (4NQO) was positive in Quillardet and Hofnung's SOS chromotest using Escherichia coli strain PQ37 both in the presence and absence of microsomal (S9) supplements and in the Salmonella typhimurium umu tester strains NM2009 and NM3009, which express high levels of O-acetyltransferase (O-AT) and O-AT plus nitroreductase (NR) respectively. m-Nitrocinnamic acid (m-NCA) was weakly positive in strains NM2009 and NM3009, but negative in the SOS chromotest; m-dinitrobenzene (m-DNB) was weakly positive in strain NM2009, intermediate positive in strain NM3009, but negative in the SOS chromotest; 2,4-dinitrotoluene (2,4-DNT) was weakly positive in strain NM3009, but negative in strain NM2009 and in the SOS chromotest. However it still showed a dose-response relationship in strain NM2009. In view of these results, it is suggested that investigators planning to screen miscellaneous nitroarenes for their genotoxicity in the future should consider taking advantage of the increased sensitivity which is conferred on S. typhimurium strains NM2009 and NM3009 by virtue of their capacity to overexpress O-AT or O-AT and NR.