Alternate feeding of glucose and propionic acid to phosphate-depleted batch cultures of Ralstonia eutropha (reclassified from Alcaligenes eutrophus) produced poly(3-hydroxybutryate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] with thermal properties markedly different from random copolymers of similar monomer content. These polymers exhibited a single glass transition and a single melting peak that was significantly higher than expected for a random copolymer. The polymers were found, by solvent fractionation and NMR, to be a mixture of poly(3-hydroxybutyrate) [P(3HB)] and a random copolymer, P(3HB-co-3HV). When compared with random copolymers of similar monomer content, the polymers produced by alternate substrate feeding displayed no improvement in mechanical properties and possessed similar aging characteristics.