Campylobacter coli strains from clinical and other sources were examined in terms of O (heat-stabile; HS) serotype and by several molecular typing techniques. Restriction fragment length polymorphism (RFLP) around the three 16S rRNA genes revealed 10 variants, none found in Campylobacter jejuni. RFLP analysis of a polymerase chain reaction amplicon generated from the flagellin gene (flaA) yielded 11 polymorphism groups, some of them linked to HS serotypes. Enlarged flaA genes, contributing three further polymorphisms, were detected in strains isolated from fresh water. Restriction of the genome with SmaI and pulsed-field gel electrophoresis was the most discriminatory typing method, detecting 33 macrorestriction profiles that subtyped within HS serotypes. The coincidence of HS serotype and the three genotypic markers identified clonal lines of evolutionary and epidemiologic significance.