Campylobacter jejuni strain 81116 contains two flagellin genes, flaA and flaB. Wild-type (WT) bacteria express flaA only, but flaB can be expressed under certain conditions. We have determined the importance of flagella for colonization of the avian caecum, which appears to be the natural environment for these bacteria. Mutants in which flaA or flaB, or both had been inactivated, and motility variants, were investigated. Flagella are not a requisite for colonization, but mutants lacking both flagellin genes colonized less efficiently than WT. Inactivation of the flaB gene, which had no effect on bacterial motility, enhanced chicken caecal colonization 1000-fold compared to WT. A variant (SF-1) with flagella composed of flagellin A, but with poor motility, also colonized better than WT. Conversely, mutants with an inactivated flaA gene colonized 100- to 1000-fold less efficiently than WT, regardless of their motility conferred by truncated or full-length flagellin B flagella. These results suggest that the presence of flagellin A, rather than motility, is essential for optimal bacterial colonization of chicken caeca.