The synthesis of poly-ß-alanine by a lipase catalyzed polycondensation reaction between ß-alanine esters is reported. The effect of different solvents, reaction temperatures and substrate/enzyme concentrations on polymer yield and degree of polymerization (DP) was determined. Also the effect of methyl substituents at the a and ß position of ß-alanine was studied. Cross-Linked Enzyme Aggregates (CLEA) of Candida antarctica lipase B converted ß-alanine esters into polymers in high yield (up to 90%) and with high (DP) (up to 53 units). Optimum results were obtained when methyl esters are used in methyl tert-butyl ether (MTBE) as the solvent, at 50°C for 16 h.