Erp (exported repeated protein) was originally characterized as a virulence factor in Mycobacterium tuberculosis and was thought to be present only in Mycobacterium leprae and members of the TB complex. Here it is shown that Erp is a ubiquitous extracellular protein found in all of the mycobacterial species tested. Erp proteins have a modular organization and contain three domains: a highly conserved amino-terminal domain which includes a signal sequence, a central variable region containing repeats based on the motif PGLTS, and a conserved carboxy-terminal domain rich in proline and alanine. The number and fidelity of PGLTS repeats of the central region differ considerably between mycobacterial species. This region is, however, identical in all of the clinical M. tuberculosis strains tested. In addition, it is shown here that a Mycobacterium smegmatis erp::aph mutant displays altered colony morphology which is complemented by all the Erp orthologues tested. The genome sequence flanking the erp gene includes cell-wall-related ORFs and displays extensive conservation between saprophytic and pathogenic mycobacteria.