This is the first report on the biosynthesis of a hitherto unknown, sulfur-containing polyester and also the first report on a bacterial polymer containing sulfur in the backbone. The Gram-negative polyhydroxyalkanoate (PHA)-accumulating bacterium Ralstonia eutropha synthesized a copolymer of 3-hydroxybutyrate and 3-mercaptopropionate, poly(3HB-co-3MP), when 3-mercaptopropionic acid or 3,3'-thiodipropionic acid was provided as carbon source in addition to fructose or gluconic acid under nitrogen-limited growth conditions. The peculiarity of this polymer was the occurrence of thioester linkages derived from the thiol groups of 3MP and the carboxyl groups of 3MP or 3HB, respectively, which occurred in addition to the common oxoester bonds of PHAs. Depending on the cultivation conditions and the feeding regime, poly(3HB-co-3MP) contributed up to 19% of the cellular dry weight, with a molar fraction of 3MP of up to 43%. The chemical structure of poly(3HB-co-3MP) was confirmed by GC/MS, IR spectroscopy, (1)H- and (13)C-NMR spectroscopy, and elemental sulfur analysis. The identification of this novel biopolymer reveals a new quality regarding the substrate range of PHA synthases and their capability for the synthesis of technically interesting polymers.