Enzymatic ring-opening polymerization of lactones was achieved by using lipase as catalyst. The polymerization of epsilon-caprolactone by Pseudomonas fluorescens lipase at 60 C in bulk for 10 days afforded a polyester with average molecular weight of 7.0 x 103. From 1H and 13C NMR analysis, the polymer possesses the terminal structure of a carboxylic acid group at one end and a hydroxyl group at the other.