Seven isolates of a slowly growing, non-chromogenic Mycobacterium species were obtained from sputum and bronchial lavage fluid samples from elderly patients in different regions of Japan. These isolates were distinguished from related non-tuberculous species by colony morphology, positive results for Tween hydrolysis, catalase at 68 degrees C, nitrate reductase and pyrazinamidase and negative results for semi-quantitative catalase, urease and arylsulfatase. The mycolic acid pattern obtained by HPLC revealed a single cluster of late-eluting mycolic acids similar to but different from those of Mycobacterium malmoense ATCC 29571(T). The 16S rRNA gene, 16S-23S internal transcribed spacer (ITS), rpoB and hsp65 sequences were unique in comparison with those of other mycobacteria. Comparison of 16S rRNA gene sequences showed that the isolates were most closely related to Mycobacterium tuberculosis H37Rv(T) (21 base differences in 1508 bp; 98.6 % 16S rRNA gene sequence similarity). A representative strain, GTC 2738(T), showed 91.9% rpoB sequence similarity with Mycobacterium marinum strain M, 95% hsp65 sequence similarity with Mycobacterium kansasii CIP 104589(T) and 81.1% 16S-23S ITS sequence similarity with Mycobacterium gordonae ATCC 14470(T). Phylogenetic analysis of concatenated sequences of the 16S rRNA, rpoB and hsp65 genes showed that strain GTC 2738(T) was located on a distinct clade adjacent to M. tuberculosis, M. ulcerans and M. marinum, with bootstrap values of 81 %. DNA-DNA hybridization demonstrated less than 70% reassociation with type strains of genetically related species and supported the novel species status of the isolates. On the basis of this evidence,