Microbial transformation of the sesquiterpene (-)-guaiol (1) [1(5)-guaien-11-ol] was investigated using three fungi, Rhizopus stolonifer, Cunninghamella elegans, and Macrophomina phaseolina. Fungal transformation of 1 with Rhizopus stolonifer yielded a hydroxylated product, 1-guaiene-9 beta,11-diol (2). In turn, Cunninghamella elegans afforded two mono- and dihydroxylated products, 1-guaiene-3beta,11-diol (3) and 1(5)-guaiene-3beta,9 alpha,11-triol (4), while Macrophomina phaseolina produced two additional oxidative products, 1(5)-guaien-11-ol-6-one (5) and 1-guaien-11-ol-3-one (6). All metabolites were found to be new compounds as deduced on the basis of spectroscopic techniques. Compounds 1-6 were evaluated for their activity against several bacterial strains.