Campylobacter jejuni strains isolated from chicken faeces and from humans suffering from gastroenteritis were used to determine the colonisation phenotypes and colonisation potential of these strains in chickens. Five different colonisation types were observed ranging from immediate and sustained colonisation to completely non-colonising. Phenotype one showed immediate colonisation with prolonged excretion of viable C. jejuni bacteria. Phenotype two showed delayed colonisation with prolonged excretion of viable C. jejuni bacteria. Phenotype three showed immediate colonisation and delayed clearing of viable C. jejuni bacteria. Phenotype four showed delayed colonisation and delayed clearing of viable C. jejuni bacteria. Strains of phenotype five could not colonise chickens. Inoculum levels to obtain maximum caecal colonisation for each phenotype for strains cultured in vitro and in vivo was also determined. Following passage in vivo through a chicken, the minimum inoculum required for sustained colonisation dropped approximately 1000-fold, however, the colonisation phenotype remained unchanged before and after a passage in vivo.