A group of campylobacters isolated from river water were found to possess unusually large flagellin genes. Both phenotype and serology were consistent with identification as Campylobacter coli. Phylogenetic analysis of small (16S, rrs) and large subunit (23S, rrl) rRNA genes of a representative strain, NCTC 13006, demonstrated high levels of relatedness with C. jejuni and C. coli (99.1 and 98.3% similarity for 16S; 99.3 and 99.4% similarity for 23S). Large flagellin proteins were demonstrated by SDS-PAGE analysis. The flaA and flaB genes were sequenced and aligned with known campylobacter flagellin amino acid sequences. The encoded FlaA protein of the new group exhibited a high degree of divergence from other Campylobacter species. Within the central variable region of FlaA, a further hypervariable domain was identified containing characteristic repeated motifs. Separate pairwise alignments performed for the variable regions of the polypeptide indicated these large fla genes were more closely related to those of C. upsaliensis than to those of C. coli or C. jejuni.