ABSTRACTCampylobacter fetussurface layer proteins (SLPs), encoded by sapAhomologues, are important in virulence. In wild-type C. fetusstrain 23D, all eight sapAhomologues are located in the 54-kb sapisland, and SLP expression reflects the position of a unique sapApromoter in relation to the sapAhomologues. The extensive homologies in the sapisland include both direct and inverted repeats, which allow DNA rearrangements, deletion, or duplication; these elements confer substantial potential for genomic plasticity. To better understand C. fetus sapisland diversity and variation mechanisms, we investigated the organization and distribution of sapAhomologues among 18 C. fetusstrains of different subspecies, serotypes, and origins. For all type A strains, the boundaries of the sapisland were relatively consistent. A 187-bp noncoding DNA insertion near the upstream boundary of the sapisland was found in two of three reptile strains studied. The sapAhomologue profiles were strain specific, and six new sapAhomologues were recognized. Several homologues from reptile strains are remarkably conserved in relation to their corresponding mammalian homologues. In total, the observed differences suggest that the sapisland has evolved differing genotypes that are plastic, perhaps enabling colonization of varied niches, in addition to antigenic variation.