Poly(3-hydoxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] was biosynthesized by Comamonas acidovorans with mixed carbon sources of n-butyric acid and 1,4-butanediol. P(3HB) and P(3HB-co-94 mol% 4HB) were obtained by fermentation with sole carbon source of n-butyric acid and 1,4,-butanediol, respectively. The content of 4HB component in P(3HB-co-4HB) increased as the composition of 1,4-butanediol in mixed carbon sources increased. These biosynthesized copolymers are not homogeneous, but mixtures of copolymers having different 4HB contents. Fractionation was carried out to obtain fractionated copolymers having a narrower distribution of composition. The change in structural and physical properties of the fractionated copolymers were investigated against the 4HB contents. Crystallinities of P(3HB-co-4HB)s were estimated using density values and the heat of fusion of P(3HB) measured was 125 J g(-1) and that of P(4HB) was 76 J g(-1). (C) 2001 Elsevier Science Ltd. All rights reserved.