Aeromonas caviae produced a random copolymer of 3-hydroxybutyric acid (3HB) and 3-hydroxyhexanoic acid (3HH) under aerobic conditions when sodium salts of alkanoic acids of even carbon numbers ranging from C-12 to C-18 and olive oil were fed as the sole carbon source. On the other hand, a copolymer of 3HB and 3-hydroxyvaleric acid (3HV) was produced by A. caviae from alkanoic acids of odd carbon numbers from C-11 to C-17. The weight-average molecular weights of P(3HB-co-3HH) were in the range (2-11) x 10(5). The structure and physical properties of P(3HB-co-3HH) with compositions of 5-25 mol % 3HH were characterized by H-1 and C-13 NMR spectroscopy, X-ray diffraction, differential scanning calorimetry, mechanical tensile measurement, and optical microscopy. The degree of X-ray crystallinity of solvent cast P(3HB-co-3HH) films decreased from 60 to 18% as the 3HH fraction was increased from 0 to 25 mol %, suggesting that 3HH units are excluded from the P(3HB) crystalline phase. The isothermal radial growth rates of spherulites of P(3HB-co-3HH) were markedly reduced with an increase in the 3HH fraction. Enzymatic degradations of P(3HB-co-3HH) films were carried out at 37 degrees C in an aqueous solution of P(3HB) depolymerase from Alcaligenes faecalis. The rates of enzymatic erosion increased markedly with an increase in the 3HH fraction to reach a maximum value at 15 mol % 3HH, followed by a decrease in the erosion rate. The above results were compared with the solid-state properties of two other microbial copolymers, P(3HB-co-3HV) and P(3HB-co-3HP) (3HP : 3-hydroxypropionic acid).