We investigated the influence of motility on Campylobacter jejuni binding and invasion of Caco-2 cells. C. jejuni was motile in soft agar at basic (pH 8.5) and neutral pH values representative of the intestinal environment. However, C. jejuni was immobilized at pH 5.0. The inability of C. jejuni to swarm on soft agar at pH 5.0 was not related to flagellar depolymerization or loss of viability. In tissue culture medium, C. jejuni displayed typical periods of straight swimming punctuated by tumbling behavior. This behavior was altered when the viscosity of the medium was adjusted to mimic the viscosity of intestinal mucus. C. jejuni showed longer periods of straight swimming with significantly increased velocity followed by pauses instead of tumbles. The binding and invasion of C. jejuni in Caco-2 cells also increased significantly in high-viscosity growth medium. We speculate that the swimming behavior of C. jejuni in a viscous environment may be an important factor in the interaction of these organisms with host epithelial cells. The pH, which affects C. jejuni motility, may also influence the tropism of these organisms.