A bacterium (strain photoB) photoassimilated 3-mercapto-2-methylpropionate as sole source of sulphur with methacrylate accumulating in the medium. This was thought to be the product of a sulphur-lyase type enzyme attacking the 3-mercapto-2-methylpropionate. Detailed examination of the biochemistry of the utilization of 3-mercapto-2-methylpropionate showed however that the thiol was first activated to 3-mercapto-2-methylpropionyl CoA. It appeared that this CoA-derivative then served as the substrate for a sulphur-lyase type enzyme, with methacrylyl-CoA as product. Further metabolism was via beta-hydroxyisobutyryl-CoA and beta-hydroxyisobutyrate.