Pseudomonas oleovorans was grown on 10-undecenoic acid and on a mixture of sodium octanoate and 10-undecenoic acid. With either or both of these substrates, cells produced polyesters containing unsaturated side chains in the repeating units. In the case of 10-undecenoic acid as the sole nutrient, the microorganism produced a polymer containing only repeating units with unsaturated side chains, A second bacterium, Rhodospirillum rubrum was grown on 4-pentanoic acid alone and it produced polyesters with shorter pendant groups in the repeating units, including both saturated and unsaturated side chains, Epoxidation of these different bacterial polyesters with m-chloroperbenzoic acid, as a chemical reagent, yielded to quantitative conversions of the unsaturated groups into epoxy groups as determined by H-1- and C-13-NMR, no side reaction was observed on the macromolecular chain by molecular weights measurements. More important, it has been possible to produce new functional bacterial polyesters containing terminal epoxy groups in the side chains, in variable proportions up to 37% by growing P. oleovorans on a 10-epoxyundecanoic acid and sodium octanoate culture feed mixture.