N-alkanoic, N-arylalkanoic and N-aryloxyalkanoic acids of 1,2-benzisothiazolin-3-one, esters and amides of N-arylalkanoic and N-aryloxyalkanoic acids and 1,1-dioxide derivatives of N-arylalkanoic acids and esters were investigated in vitro antimicrobial activity. N-arylalkanoic and N-aryloxyalkanoic acids (compounds 4-12) and their esters and amides (compounds 13-26) exhibited a good antimicrobial activity against Gram positive bacteria, with several compounds showing potencies 10-20 times higher than 1,2-benzisothiazolin-3-one. None of the chemicals tested inhibited the growth of E. coli. Yeasts and moulds possess a considerable susceptibility to compounds 12-23. The logP (octanol-water) of esters and amides of N-arylalkanoic and N-aryloxyalkanoic acids were measured by the shake-flask technique and the potencies against Gram positive bacteria of the compounds tested was related to their lipophilicity. QSAR analysis showed a bilinear relation, with a logD0 around 3 for the activity on B. subtilis. The phenoxyacetic and phenoxybutyric acid derivatives are positive outliers, showing a potency higher than that predicted from their lipohilicity. The most active compounds were further tested against different Gram positive bacteria and moulds, including Bacilli, Sarcina lutea, Staphylococcus epidermidis, Candida spp. and dermatophytes. The antibacterial and antifungal activity was specific for 1,2-benzisothiazolin-3-ones, the corresponding 1,1-dioxide derivatives being inactive. The genotoxic properties of the compounds studied were evaluated by the Bacillus subtilis rec-assay and Salmonella-microsome test. None of the compounds showed DNA-damaging or mutagenic activity.