During the past decade a considerable amount of attention has been focused upon the pathogenic importance of so-called atypical mycobacteria. Many of these organisms, such as mycobacteria of the Battey type, the "yellow-bacilli" (photochromogens), Mycobacterium scrofulaceum, and M. fortuitum have been found capable of causing pulmonary and lymphatic infections virtually indistinguishable from those due to M. tuberculosis. Others of the atypical mycobacteria give rise to skin lesions which mimic primary inoculation tuberculosis, lupus vulgaris, tuberculosis verrucosa cutis, scrofuloderma, or nonspecific ulceration. Among the latter, 2 mycobacteria stand out as well-established species: M. ulcerans, described in Australia in 1948 by MacCallum, Tolhurst, Buckle, and Sissons,1 and M. balnei, isolated in Sweden in 1951 and described in detail by Linell and Norden in 1954.2 It is reasonable to assume that these organisms are ubiquitous, as shown by reports of M. ulcerans infections in Mexico and the Congo3,4 and by