The anionic ring-opening polymerization of 3-methyl-2-azetidinone (3) in a mixture of N,N-dimethylacetamide with lithium chloride proceeded quantitatively in a homogeneous phase at 25 degrees C, as well as the living anionic polymerization of 3,3-dimethyl-, 4,4-dimethyl-2-azetidinone (1 and 2, respectively) in a similar condition. However, the molecular weight dispersion of the polyamide obtained from 3 was found to be higher than that obtained from 1 and 2. The aminolysis reaction of their N-benzoyllactams and N-acyllactams corresponding to their growing species with benzylamine was investigated kinetically, and one of the reasons for broadening of the molecular weight distribution of the polyamide obtained in the anionic polymerization of 3 was speculated to result from a low value of the ratio of the initiation reaction constant to the propagation one.