10-Ketostearic acid was unexpectedly observed during bioconversion of oleic acid to 15-, 16-, and 17-octadecenoic acids by Bacillus pumilus. The unexpected conversion was caused by contaminants which were isolated, characterized, and identified, The three isolates were Gram positive cocci that grew anaerobically and were sensitive to furazolidone and lysostaphin, These characteristics suggested that the isolates belonged to the genus Staphylococcus. Physiological and biochemical characterization, fatty acid profiling, and DNA reassociation determinations indicated that the isolates were strains of the species Staphylococcus warneri. The organisms were deposited in ARS Culture Collection as NRRL B-14932, NRRL B-14933, and NRRL B-14934.