Bacteria possess multiple mechanisms for the transport of metal ions. While many of these systems may have evolved in the first instance to resist the detrimental effects of toxic environmental heavy metals, they have since become adapted to a variety of important homeostatic functions. The 'P'-type ATPases play a key role in metal ion transport in bacteria. A Cu+-ATPase from the intracellular bacterium Listeria monocytogenes is implicated in pathogenesis, and similar pumps in Mycobacterium tuberculosis and M. leprae may play a comparable role. Intracellular bacteria require transition metal cations for the synthesis of superoxide dismutases and catalases, which constitute an important line of defence against macrophage-killing mechanisms. The macrophage protein Nramp1, which confers resistance to a variety of intracellular pathogens, has also been shown recently to be a divalent amphoteric cation transporter. Mycobacterial homologues have recently been identified by genomic analysis. These findings suggest a model in which competition for divalent cations plays a pivotal role in the interaction between host and parasite.