Biodegradable polyesters were synthesized by ring-opening copolymerization of L-lactide (LA) with DL-beta-methyl-delta-valerolactone (MV). H-1 and C-13 nuclear magnetic resonance analyses showed that these copolymers were statistical and that their number-average molecular weights were within the range of (3-5) x 10(4). The melting temperatures of these polyesters were dependent upon the LA content in the copolymers, thus showing that high LA composition resulted in higher T-m. The copolymers containing more than 90 mol% LA formed tough and hard films, whereas those with less than 80 mol% LA formed flexible films similar to natural rubber. Accelerated hydrolysis was carried out with copolymers of various compositions at 70 degrees C for 18 days. The hydrolysis rates were much faster than that of poly(LA), which is well known to be a hydrolysable polymer. The copolymers were easily hydrolysed with lipases from Rhizopus arrhizus, R. delemar, Pseudomonas sp. and Candida cylindracea. In particular, the lipases from R. arrhizus and R. delemar proved to be very effective in promoting hydrolysis.