Poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-6hydroxybutyrate) [P(3HB-co-3HV-co-4HB)] terpolymers were produced in fed-batch cultures of Ralstonia eutropha (reclassified from Alcaligenes eutrophus) using glucose, propionic acid and either 4-hydroxybutyric acid or gamma-butyrolactone as carbon sources. The thermal and mechanical properties of terpolymers containing different proportions of the constituent monomers were characterized. Most of the terpolymers possessed low melting temperatures and the majority possessed crystallization half-times between 4 and 16 min, compared with 0.5 min for poly(3-hydroxybutyrate) [P(3HB)]. For certain polymer compositions, containing approximately 10 mol% of both 3-hydroxyvalerate (3HV) and 4-hydroxybutyrate (4HB), the mechanical properties were superior to those of P(3HB-co-0-30% 3HV) range of polymers and did not display any signs of deterioration over a one month test period. A compression molded polymer film containing 78 mol% 3HB, 11 mol% 3HV and 11 mol% 4HB was found to possess an elongation to break of 700% after one month, compared with 740% after molding, but ultimately declined to 35% after 13 months. The aging process was found to depend upon the primary crystallization rate. (C) 2000 Elsevier Science Ltd. All rights reserved.