The enteric bacteria Salmonella typhimurium has the ability to invade (enter) nonphagocytic cells. The internalization process occurs as a result of an intimate interaction between the bacteria and the host cell, in which S. typhimurium triggers a cascade of host cell-signaling events leading to the formation of host cell membrane ruffles and bacterial uptake. Using high resolution scanning electron microscopy, we have observed that contact with cultured epithelial cells results in the formation of appendages on the surface of S. typhimurium. The formation of such appendages did not require de novo protein synthesis, and it was transient, since these surface structures were no longer present on bacteria that had initiated the internalization event. Salmonella mutants defective in the transient formation of these surface organelles were unable to enter into cultured epithelial cells, indicating that such structures are required for bacterial internalization.