Phenol-phthiocerol glycolipids have been found previously in Mycobacterium leprae, M . kansasii, M. bovis and M . marinum, but not in M . tuberculosis. A search for such glycolipids in this latter species showed that the Canetti strains of M . tuberculosis synthesize a major triglycosyl phenol-phthiocerol, accompanied by minor amounts of other glycolipids with a similar aglycone moiety.The triglycoside moiety has the following structure: 2,3,The aglycone moiety consists in phenol-phthiocerol (two homologs). Its two secondary alcohol functions are esterified by mycocerosic acids (homologs with 26-32 carbon atoms and with 2 -4 methyl branches). The proposed structure differs on several points from the M . leprae glycolipids, but presents some analogy with the major glycolipid of M. kansasii.A minor monoglycosyl phenol-phthiocerol was also studied. Its overall structure is very similar to that of M . bovis, with 2-0-methyl rhamnose as sugar moiety.In the last few years, it has been realized that glycolipids are major surface antigens of pathogenic mycobacteria [l], and important applications are expected from this: the detection and identification of bacteria in patients and the elaboration of semi-synthetic vaccines.Three phenol-phthiocerol trisaccharides isolated from Mycobacterium leprae have attracted much attention, since they were the first antigens to be chemically defined in this important pathogen [l -31.Long ago, phenol-phthiocerol monosaccharides were identified in M . bovis [4], M. marinum [5] and M . kansasii [6]; the structure of the glycolipid of the latter species has been revised recently [7, 81. The four mycobacteria quoted above, as well as M . tuberculosis, are pathogenic for man and have the same kind of mycolic acids in common [9]. However no phenol-phthiocerol glycolipid had been found previously to this paper in the most important pathogen of this genus, M . tuberculosis. We searched for glycolipids in various strains belonging to this species, and a major phenol-phthiocerol trisaccharide, accompanied by minor amounts of a phenol-phthiocerol monosaccharide, was detected in the four Canetti strains of M. tuberculosis tested. The structure of the triglycosyl glycolipid is described here. It differs on several points from the glycolipids of M . leprae, and presents some analogy with the major glycolipid of M. kansasii.