The DNA-DNA homology percentages obtained in this study indicate that M. avium and M. paratuberculosis belong to one species. Consequently, M. paratuberculosis ought to be considered a variant of M. avium, and the following designations are proposed: Mycobacterium avium, subsp. avium. Mycobacterium avium, subsp. paratuberculosis. Identification and classification of "wood pigeon mycobacteria" occurring in wild animals have been problematic due to their dysgonic and mycobactin-dependent growth. DNA-DNA homology percentages indicate that these bacteria are closely related to reference strains both of M. avium and of M. paratuberculosis. "Wood pigeon mycobacteria" should therefore be classified as atypical strains of M. avium, and the following designation is proposed: Mycobacterium avium subsp. columbae.