A numerical classification using fifty-two characters was done on 72 strains of mycobacteria, including Mycobacterium avium, M. terrae, the pathogenic nonphotochromogens and the pathogenic scotochromogcns.The results indicated that the test strains could be divided into two subgroups, the first consisting of M. avium and M. terrae and the second of the nonphotochromogens and the scotochromogens. Distinguishing characters for differentiation between these organisms were described, and taxonomic position of these organisms was discussed.Runyon [14] grouped so-called "atypical" mycobacteria into four groups: Group I, photochromogens; group II, scotochromogens; group III, nonphotochromogens; and group IV, rapid growers. Among these, group I is now recognized to consist of the species Mycobacterium kansasii [6,20] and M. marinum (M. balnei), and group IV consists of M. fortuitum. However, the taxonomy of the group II scotochromogens and the group III nonphotochromogens requires more study.An approach to this problem using numerical classification [16,17,19] was undertaken by Bojalil, CerbOn and Trujillo [1]. They divided the group II scotochromogens into three subgroups M. flavescens, M. gordonae and M. marianum, and considered group III nonphotochromogens as identical to M. avium. Because their classification was done using much fewer characters than proposed by Sneath [18], their results merit further investigation. Recently Tsukamura [25,27] did a numerical classification of mycobacteria using approximately ninety characters and found that M. terrae, M. avium, the nonphotochromogens and the scotochromogens formed a large cluster which could be separated into three subclusters; (1) M. terrae, (2) M. avium and part of the nonphotochromogens, and (3) the remaining part of the nonphotochromogens and the scotochromogens. Tsukamura and Tsukamura [29] adopted the same method to classify slow-growing mycobacteria using forty-three characters obtained by omitting characters showing negative matches among the slow-growing mycobacteria. The results obtained by this method were the same as those obtained previously. Wayne [32] analysed the nonphotochromogens using twenty-five characters and reported that the Battey bacilli (pathogenic nonphotochromogens) should be named as 233