The in vitro antimicrobial activity of bz-nitro-, 3-ethylacetate-, 3-amino- and 3-substitutedamino 2,1-benzisothiazoles was evaluated. The compounds studied were found to display a very low activity against bacteria and fungi, with the exception of compound 61, which exhibited a relatively high activity against Bacillus subtilis, Escherichia coli and Saccharomyces cerevisiae. As for genotoxic properties, compounds 1-3, 5 and 6 showed DNA-damaging activity in the Bacillus subtilis rec-assay. The Salmonella-microsome assay confirmed the genotoxicity of these compounds and also revealed the mutagenicity of compounds 4, 7-12, 23, 24, 31, 33-35, 38, 39, 44, 49, 51, 53, 57-63. Structure-activity relationships showed all the compounds containing an aromatic nitro group or an unsubstituted amino group to possess genotoxic properties. Whereas most of the 3-acylamino-, 3-acylalkylamino- and 3-azomethynderivatives showed mutagenic activity, none of the 3-alkylamino-2,1-benzisothiazoles was active. None of the 1,2-isomers studied showed genotoxic properties.