Lipases from porcine pancreas and from Pseudomonas cepacia catalyze the ring-opening polymerization of lactones. Polymerization of the four-membered beta-butyrolactone yielded poly(3-hydroxybutyrate) in up to 89% yield with a degree of polymerization of 3-12. Polymerization of the four-membered beta-propiolactone, the five-membered gamma-butyrolactone, and the seven-membered epsilon-caprolactone also yielded the corresponding polyesters in good yields with degrees of polymerization up to 25. Coordination catalysts, such as alumoxanes, fail in the homopolymerization of gamma-butyrolactone; thus this lipase route ill the first homopolymerization of gamma-butyrolactone. To characterize the product polyesters, we used the method of MALDI-TOF mass spectroscopy to determine both the true molecular weight of the components and the mass of the repeat unit.