The Fur (ferric uptake regulator) protein is a master regulator of iron metabolism in gram-negative bacteria. In the present study, the effect of a partial deletion of the fur gene on the acid-tolerance response and in vivo virulence of avian Escherichia coli was examined. The fur mutant was unable to trigger the acid-tolerance response as observed in the wild-type parent strain. However, the mutant was as virulent as the wild-type parent strain when tested in 1-day-old chickens by subcutaneous inoculation. These data indicate that the fur gene is involved in the acid-tolerance response but not involved in the virulence of E. coli, as detected by the ability to cause septicemia in our experimental infection.