Mycobacterium tuberculosis complex isolates recovered from goats were originally classified as Mycobacterium tuberculosis subsp. caprae; however, this subspecies was recently reclassified as Mycobacterium bovis subsp. caprae. Besides biochemical (sensitivity to pyrazinamide) and epidemiological features, strains of this unusual member of the M. tuberculosis complex show a special combination of pncA, oxyR, katG and gyrA gene polymorphisms. Sequence analysis of the gyrB gene in these strains reveals special nucleotide substitutions not found in other members of the M. tuberculosis complex that can be used to differentiate caprine mycobacterial strains from M. bovis and other members of the M. tuberculosis complex. M. tuberculosis subsp. caprae now appears not to be restricted to Spanish goats, as strains of this organism have been isolated from cattle, wild boar and pigs. Its occurrence has also been reported in France, Austria and Germany. Two studies on the evolution of the M. tuberculosis complex based on the presence/absence of regions of difference have shown that the group of caprine isolates (or its ancestor) is older than M. bovis (or its ancestor). These findings reinforce the original suggestion that the caprine mycobacterial strains are a taxon of the M. tuberculosis complex, independent of M. bovis. Within the current context of the existing nomenclature of the M. tuberculosis complex, it is proposed that M. tuberculosis subsp. caprae be elevated to species status, as Mycobacterium caprae comb. nov., sp. nov.