ABSTRACTCampylobacter jejuniis an enteropathogen for humans but commensal for chickens. In both hosts, the flagella and motility are important colonization factors. The flagellin gene is duplicated inCampylobacter, but only one flagellin gene,flaA, is sufficient for motility. We found that, during colonization of the chicken intestine, a nonmotile flaAmutant of C. jejuniunderwent rearrangements within its flagellin locus, thereby regaining its motility and colonization capacity. In contrast, in vitro motile revertants isolated from liquid culture showed different flagellin DNA rearrangements than after reversion in the chicken.