Poly(succinimide) was synthesized from L-aspartic acid at 260 degrees C for 6 h, and the microstructures of the polymer were analyzed in detail using H-1 and C-13 NMR spectroscopy. Several end groups, irregular structures, and byproducts were identified and quantified. Although C-termini such as dicarboxylic acid end groups and succinimide end groups occupy 10% of the monomer units in the polymer, an amino end group is not detected because it is probably reduced by the deamination which produces many irregular structures possessing a maleic or fumaric acid unit. The polymer chain is preferentially extended by the condensation of the amino group of a monomer and the dicarboxylic acid end group of the polymer. The presence of branched units and ring open amide units are shown by the results of DQF-COSY, a comparison of the amide proton signals, and the disparity between the sums of C- and N-termini. The stereoregularity of the investigated poly(succinimide) is low.