Variation in growth rate has provided a basis for the broad classification of Mycobacterium: (i) the slow-growing class includes the human pathogens Mycobacterium tuberculosis and M. leprae, and (ii) the fast-growing class includes saprophytic nonpathogens, such as M. smegmatis. An intimate association between DNA chromosomal replication and growth rate have been suggested. However, the molecular basis of this relation is unknown. In this article, we summarise our recent work on the study of the origin replication region of some species of mycobacteria, determination of the factors regulating the initiation of DNA replication and the characterisation of the main factor of the replicative machinery, the DnaA protein.