Salmonella typhimurium grown under oxygen-limiting conditions were found to enter into, elicit actin filament rearrangement in, and effect morphological changes upon HEp-2 cells within 15 min after infection. Video microscopy revealed that host cell morphological changes associated with entry began within 1 min of productive adherence. Polarized Caco-2 cell morphology was affected 40 s after infection with low-oxygen-grown S. typhimurium. Stationary-phase S. typhimurium did not elicit these phenomena within this time-period even when adherence was enhanced with the afimbial adhesin, AFA-I. Thus, environmental cues regulate S. typhimurium invasion factors, allowing for immediate entry into host cells. Additionally, actin filament rearrangement and morphological changes in the eukaryotic host cell are essential for entry and occur within minutes of infection.