Living anionic polymerization of two kinds of methyl-substituted beta-lactams, 3,3-dimethyl-, and 4,4-dimethyl-2-azetidinones, was attained at 25 degrees C in N,N-dimethylacetamide containing 5 wt % of lithium chloride and proceeded in a homogeneous phase quantitatively. The resulting polyamides were found to have a narrow molecular weight distribution from the gel permeation chromatography. The number average molecular weights estimated from the peak intensities in the H-1-NMR spectra were almost equal to those calculated from the mol ratio of the consumed lactam to the corresponding N-benzoyl derivative used as an activator. In addition, the acyllactam-type growing chain ends were modified quantitatively by the reaction with benzylamine after the polymerization.