Eleven different types of bacteria were isolated which were capable of growing on ε-caprolactam, the monomeric material for nylon 6 polyamide, as the sole source of both carbon and nitrogen. The optimal concentration of ε-caprolactam for the bacterial growth was about 0.6% in a synthetic liquid medium enriched with a small amount of yeast extract. The bacterial strains grew also on γ-butyrolactam, δ-valerolactam and the ω-amino acids corresponding to these lactams and ε-caprolactam. Ammonium adipate was a good substrate for the growth of all the strains. One strain of Corynebacterium aurantiacum was found to be capable of utilizing cyclic and linear oligomers of 6-aminocaproic acid with an exception of cyclic dimer. The strains of corynebacteria required vitamin B1 for growth. Metabolism of ε-caprolactam and related compounds is discussed briefly.