Saccharomyces cerevisiae NCYC 73 biotransforms various organosulfides to equivalent sulfoxides. Both the product yield and stereoselective of the biooxidation are influenced profoundly by the structure of the organosulfide substrate. In each case the predominant enantiomer produced is the R-(+)-sulfoxide. No significant further oxidation of sulfoxide to sulfone was recorded with NCYC 73.