Morphologic properties of colonies growing on either 7H10 or corn meal glycerol agar plates, or both, are of assistance in identification of mycobacterial pathogens. By examination of microscopic colonies, early reporting of Mycobacterium tuberculosis may be made, and common difficulties from contaminated cultures can be avoided. Colonies of M. kansasii have a distinctive SR morphology and exhibit crystal production as well as photochromogenicity. Colonies of Battey bacilli are usually sharply distinct, being thin, smooth, and translucent. M. xenopi is recognized by slowly developing yellow colonies which, by microscopic examination, can be seen to be surrounded with projecting filaments. M. abscessus can most easily be distinguished from the other common rapid grower, M. fortuitum, by the absence of branching filamentous extensions from its colonies.