Bacteriophage infection requires a specific interaction with the outer surface of a bacterial host followed by interaction with the cell membrane and phage DNA injection. Phages of the mycobacteria encounter a cell wall that is rich in unusual lipid- and sugar-containing components which form a formidable barrier that must be passed to gain access to the membrane. We describe here a gene of Mycobacterium smegmatis that confers resistance to mycobacteriophages L5 and D29. The phage-resistance phenotype results not from mutation but from elevated expression of a wild-type gene. It appears that the product of this multicopy phage-resistance (mpr) gene may alter the structure of the host cell wall or membrane, thereby inhibiting productive phage DNA injection.