Selected strains of the bacterium Pseudomonas putida (previously shown to effect dioxygenase-catalysed asymmetric cis-dihydroxylation of alkenes) have been found to yield chiral sulfoxides from the corresponding sulfides with a strong preference for the (R)or (S)-configurations but without evidence of sulfone formation; similar results obtained using an Escherichia coliclone (pKST11, containing the Tod C1 C2 B and A genes encoding toluene dioxygenase from P. putida NCIMB 1 1767) are again consistent with a stereoselective dioxygenase-catalysed su Ifoxidation.