The enzymatic ring-opening copolymerization of epsilon-caprolactone (epsilon-CL) and beta-lactam by using Candida antarctica lipase B (CAL-B) as catalyst was studied. Variation of the feed ratios of 25:75, 50:50, and 75:25 of epsilon-CL/beta-lactam was performed. The products contain poly(epsilon-CL-co-beta-lactam) and the homopolymers of poly(epsilon-CL) and poly(beta-lactam). The structure of the copolymers was determined by MALDI-ToF MS. Poly(epsilon-CL-co-beta-lactam) has an alternating and random structure consisting of alternating repeating units with oligo(epsilon-CL) or oligo(beta-lactam). The highest fraction of the alternating copolymers resulted from the reaction with a feed ratio 50:50. The copolymer is a semicrystalline polymer with a T-m at 124 degrees C and T(g)s at -15 and 50 degrees C. Interestingly, the copolymer also demonstrated cold crystallization at 29 and 74 degrees C, after quenching the sample from the melt in liquid nitrogen.