Spore suspensions of P. roqueforti metabolized sodium palmitate into carbon dioxide and a variety of carbonyl compounds, including a homologous series of methyl ketones. Both D-glucose and L-proline suppressed the catabolism of palmitate to COS but stimulated the rate of formation of carbonyl compounds, including methyl ketones. Spore concentrations of 6.3 X lo8 spores/ml produced the optimum yield of (32.4%) carbonyl compounds from the incubation of 5 m M of palmitate in the presence of 20 m M of D-glucose under optimum conditions of pH and tempera-ture; i,e., pH 6.5 (0.1 M phosphate buffer) and 30". Analyses of carbonyl compounds formed from [U-14C]palmitate revealed that a homologous series of labeled methyl ketones, C3 to C15 inclusive, was produced. Pentadecanone contained the highest radioactivity, followed by tridecanone and undecanone, with the lower methyl ketones containing varying amounts of radioactivity. Appreciable amounts of carbonyl compounds other than methyl ketones (Le., labeled, saturated, and unsaturated aldehydes) were also produced from [14C]palmitic acid.