Two-component regulatory proteins function in bacteria as sensory and adaptive factors in response to a wide range of environmental stimuli. Some two-component systems, such as PhoP/PhoQ, control transcription of key virulence genes essential for survival in host cells in diverse intracellular bacterial pathogens, including Salmonella sp., Shigella sp. and Yersinia sp. In this study, we have disrupted the phoP gene from Mycobacterium tuberculosis, which codes for a putative transcription regulator factor of the two-component system PhoP/PhoR. The phoP mutant strain exhibited impaired multiplication when cultured in mouse bone marrow-derived macrophages. However, the mutation did not appear to affect survival of the organisms adversely inside macrophages. The mutant strain was also attenuated in vivo in a mouse infection model, with impaired growth observed in the lungs, livers and spleens. The results suggest that the phoP gene is required for intracellular growth of M. tuberculosis but is not essential for persistence of the bacilli.