Enzymatic ring-opening copolymerization of lactones was carried out by using Pseudomonas fluorescens lipase in bulk. In the copolymerizaton of ε-caprolactone (ε-CL) and δ-valerolactone at 60°C for 10 days a copolymer with molecular weight of 3.7×10³ was obtained and its structure was found to be of random structure having both units from ¹³C NMR anlaysis. Effects of the feed ratio, the reaction time and temperature on the copolymerization have been examined. The enzymatic copolymerization of ε-CL with other lactones and D-lactide also gave corresponding copolymers.