The genus Mycobacterium , which is a member of the high G+C group of Gram-positive bacteria, includes important pathogens, such as M. tuberculosis and M. leprae . A recent publication in PNAS reported that M. marinum and M. bovis bacillus Calmette–Guérin produce a type of spore known as an endospore, which had been observed only in the low G+C group of Gram-positive bacteria. Evidence was presented that the spores were similar to endospores in ultrastructure, in heat resistance and in the presence of dipicolinic acid. Here, we report that the genomes of Mycobacterium species and those of other high G+C Gram-positive bacteria lack orthologs of many, if not all, highly conserved genes diagnostic of endospore formation in the genomes of low G+C Gram-positive bacteria. We also failed to detect the presence of endospores by light microscopy or by testing for heat-resistant colony-forming units in aged cultures of M. marinum . Finally, we failed to recover heat-resistant colony-forming units from frogs chronically infected with M. marinum . We conclude that it is unlikely that Mycobacterium is capable of endospore formation.