The metabolite production of the gram positive bacterium Staphylococcus xylosus when cultivated in a defined medium containing 18 amino acids, 6 vitamins and 2 purines was characterised. Several compounds not previously reported as metabolites of this organism were identified including 2,5-methylpyrazine, 2-phenylethylacetate, 2-methyltetrahydrothiophen-3-one, 3-(methylthio)-propanoic acid and 3-(methylthio)-propanal. The organoleptic metabolites derived from branched-chain amino acid catabolism; 2-methylpropanal from valine, 2-methylbutanal from isoleucine and 3-methylbutanal from leucine were detected at levels ranging from 0.4 to 2.0 muM. The concentrations of the corresponding carboxy acids were 963, 858 and 1486 muM respectively. We demonstrated that a-ketoisocaproic acid was biotransformed to 3-methylbutanal which immediately was oxidised into 3-methylbutanoic acid. Kinetic studies of the conversion of 2-methylpropanal, 2-methylbutanal and 3-methylbutanal into their corresponding acids were also performed and we found K-m values of 0.8, 0.9 and 1.6 muM, and V-max values of 9.4, 7.1 and 5.9 mumol/min/10(12) CFU, respectively.