The four-membered beta-butyrolactone (BL) was polymerized using a ring-opening polymerization reaction with lipase as a catalyst; at a temperature between 60 and 100 degrees C to yield the corresponding cyclic and linear poly(3-hydroxybutyrate)s [P(3HB)] with weight-average molecular weights of up to 7300. Among the tested lipases, porcine pancreatic lipase (PPL) and Candida cylindracea lipase (CC) showed the best results with respect to the molecular weight of the resulting polymer and the monomer conversion. It was found that a significant amount of the cyclic P(3HB) fraction in the resulting polymer was produced and the cyclic polymer was increased with increasing monomer conversion. The (R)-BL was more easily polymerized by lipase to form the corresponding P(3HB) having a higher molecular weight than that formed by using (R,S)-BL.