SummaryDNA probes were used to identify restriction‐fragment‐length polymorphisms (RFLPs) in DNA samples, demonstrating that theMycobacterium aviumcomplex could be clearly divided IntoM. aviumandMycobacterium intracellularestrains. Less than 2% DNA base substitution was found betweenM. aviumstrains, whereas theM. intracellularestrains had greater than 15% base substitution. The Johne's disease bacillus,Mycobacterium paratubercuiosis(American type strain), was found to be distinguishable from theM. aviumcomplex serotypes examined. Strain 18 was found to be identical toM. avium.The rat leprosy bacillus,Mycobacterium lepraemurium, was found to be very closely related, but not identical, toM. av