From soil, two strains of Burkholderia sp. were isolated that synthesized and accumulated a copolyester of 3-hydroxybutyric acid and 3-hydroxy-4-pentenoic acid from single, unrelated carbon sources such as surcose or gluconate. Strain IPT77B, for instance, accumulated from gluconate poly(3-hydroxybutyric acid co-3-hydroxy-4-pentenoic acid) up to 70% (w/w) of the cellular dry matter, and 3-hydroxy-4-pentenoic acid contributed up to 6.9 mol 100/mol constituents. The occurrence of the constituent with the vinyl pendant group in the polyester was confirmed by gas-chromatographic and nuclear magnetic resonance analysis.