In accordance with Recommendation 30b of the International Code of Nomenclature of Bacteria, which calls for the development of recommended minimal standards for describing new species, we propose minimal standards for describing the genus Mycobacterium and new slowly growing species of this genus. The minimal standards for assignment of a strain to the genus Mycobacterium include acid-alcohol fastness, a DNA G+C content in the range from 61 to 71 mol%, and mycolic acid detection with characterization of C22 to C26 pyrolysis esters. The recommended minimal standards for describing a new slowly growing Mycobacterium species are based on the results of phenotypic and genomic studies and include the results of the following conventional tests: growth at 25, 30, 33, 37, 42, and 45 degrees C; pigmentation; resistance to isoniazid, thiophene-2-carboxylic acid hydrazide, hydroxylamine, p-nitrobenzoic acid, sodium chloride, thiacetazone, picrate, and oleate; catalase activity; Tween hydrolysis; urease activity; niacin detection; and nitrate reductase, acid phosphatase, arylsulfatase, pyrazinamidase, and alpha-esterase activities. In addition, a mycolic acid profile should be determined, and DNA-DNA hybridization experiments in which the difference between the denaturation temperature of the homologous reaction and the denaturation temperature of the heterologous reaction is determined should be performed. This proposal has been endorsed by the members of the Subcommittee for Taxonomy of the Mycobacteria of the International Committee on Systematic Bacteriology.