A selection strategy was devised to identify bacterial genes preferentially expressed when a bacterium associates with its host cell. Fourteen Salmonella typhimurium genes, which were under the control of at least four independent regulatory circuits, were identified to be selectively induced in host macrophages. Four genes encode virulence factors, including a component of a type III secretory apparatus. This selection methodology should be generally applicable to the identification of genes from pathogenic organisms that are induced upon association with host cells or tissues.