A hitherto unknown copolymer that contains sulfur in the backbone linking 3-hydroxybutyrate and 3-mercaptobutyrate by thioester linkages, poly(3HB-co-3MB), was synthesized by the polyhydroxyalkanoate(PHA-) accumulating bacterium Ralstonia eutropha, when 3-mercaptobutyric acid was fed as the carbon source in addition to gluconate. The chemical structure of this novel polymer was confirmed by gas chromatography/mass spectrometry, infrared spectroscopy, H-1 and C-13 nuclear magnetic resonance spectroscopy, and elemental sulfur analysis. Different poly(3HB-co-3MB) samples were isolated and characterized, and the molar 3MB fraction contributed up to 72 mol %. Regarding the chemical structure of 3-mercaptobutyrate, which is analogous to 3-hydroxybutyrate, the incorporation of this novel polymer constituent represents a second example of 3-mercaptoalkanoates that are incorporated into biopolymers by the PHA synthase of R. eutropha.