The abilities of three Pseudomonas strains transform each of the six isomers of methylbenzothiophene was studied. These compounds would not support growth but were biotransformed in a cell suspension of one of the violates and in cultures of the other two isolates while they grew on 1-methylnaphthalene or glucose. Sulfur-containing metabolites were identified by GC/MS and GC/FTIR, and when possible, by comparison with authentic standards. Sulfoxides and sulfones were frequently detected and were the most abundant products from 2- and 3-methylbenzothiophene. Two of the isolates oxidized the methyl groups to carboxylic acids which were abundant products from 3-, 5-, and 6-methylbenzothiophenes