We studied the taxonomic positions of the rapidly growing organism Mycobacterium fortuitum and phenotypically related organisms. We confirmed that "Mycobacterium peregrinum" ATCC 14467T (T = type strain) is genetically independent of M. fortuitum ATCC 6841T by using various DNA hybridization conditions. Strains that were genetically identified as "M. peregrinum" were phenotypically differentiated from M. fortuitum ATCC 6841T. Thus, we propose that "M. peregrinum" should be revived as an independent species, Mycobacterium peregrinum sp. nov., nom. rev. The type strain is strain ATCC 14467. M. fortuitum subsp. acetamidolyticum ATCC 35931T exhibited a high level of DNA relatedness to M. fortuitum ATCC 6841T. The hybridized DNAs maintained stable heteroduplexity at high stringency; thus, we confirmed that M. fortuitum subsp. acetamidolyticum is identical to M. fortuitum ATCC 6841T. We found that M. chelonae subsp. abscessus ATCC 19977T is genetically different from M. chelonae subsp. chelonae NCTC 946T on the basis of the results of quantitative hybridization even under optimal conditions. There was no reason to maintain this organism as a subspecies of M. chelonae. Thus, we propose that M. chelonae subsp. abscessus should be elevated to species status as Mycobacterium abscessus (Kubica et al.) comb. nov. The type strain is strain ATCC 19977.