This paper describes in vitro evaluation by Bacillus subtilis rec-assay and Salmonella-microsome assay of the genotoxic properties of some 1,2-benzisothiazole and 1,2-benzisothiazolin-3-one derivatives, containing groups (aromatic nitro, aromatic amino, epoxide groups) often associated with mutagenic properties and defined as "alerting groups". Compounds 2, 3, 4, 7, 8, 9, 11, 14, 16, 20, 21 and 22 exhibited DNA-damaging activity. The Salmonella-microsome assay confirmed the genotoxic activity of these compounds, which was also demonstrated for compounds 5, 6, 10, 12, 13, 15, 17, 18, 19, 25 and 27. Relationships between chemical structure and genotoxic activity showed that all the 1,2-benzisothiazole derivatives containing an alerting group were active. On the contrary, among 1,2-benzisothiazolin-3-one derivatives, compounds 24 and 26 were non-mutagenic, although an alerting group was present in the molecule.