The atp and fur genes are involved in the acid tolerance response of Salmonella typhimurium. An atp::Tn10 mutant was avirulent in the mouse typhoid model when assayed by oral and intraperitoneal routes. However, a fur mutant was completely virulent by the intraperitoneal route. No relevant differences in intracellular survival or invasion rates were observed for the two mutants in macrophages and epithelial cells. These data indicate that separate acid tolerance response genes may have different roles in S. typhimurium virulence.