A series of P(3HB-co-3MP)s with different 3MP unit content was biosynthesized by the fermentation of W. eutropha in a medium containing sodium gluconate and DTDP as carbon sources at different pH conditions ranging from pH 6.0 to 8.0. The P(3HB-co-3MP) samples were fractioned using the solvent/nonsolvent mixed solvent chloroform/heptane and the comonomer unit composition was investigated. It was found that W. eutropha produces P(3HB-co-3MP)s with extremely different 3MP unit content ranging from 3.6 to 70.0 mol-%, depending on the pH value of the fermentation medium. The copolyester samples produced in mild basic medium have a considerably narrower compositional distribution than the samples from acidic medium. The highest polymer yield was obtained at pH 8.0.DSC diagram for P(3HB-co-3MP)s biosynthesized in different pH medium. [graph: see text] DSC diagram for P(3HB-co-3MP)s biosynthesized in different pH medium.