High molecular weight poly(succinimide) was quantitatively synthesized by polycondensation of L-aspartic acid using a catalytic amount of o-phosphoric acid in the mixed solvent of mesitylene and sulfolane. In addition, under similar conditions, L-aspartic acid and omega-amino acid produced an organic solvent-soluble poly(succinimide-co-omega-amino acid), which melted below the decomposition temperature.