Diethyl L-aspartate was polymerized by proteinase at a temperature between 4 and 50 degrees C to yield poly(ethyl alpha,beta-L-aspartate). Bulk polymerization of diethyl L-aspartate, especially at temperatures between 40 and 50 degrees C, and preferably using alkalophilic proteinase with the addition of a small amount of water gave poly(ethyl L-aspartate) as a white powder with a molecular weight of up to 3600 and having about 88% alpha-linkages.