The principal aim during this study was to develop appropriate in vitro and in vivo systems to examine colonisation mechanisms of E. coli O157:H7 in the bovine GIT. An adherence assay on cultured tissue explants was developed to compare the ability of different aspects of E. coli O157:H7 adherence. In two separate experiments the contribution of intimate attachment, thought to be essential for virulence in humans, was assessed. The ability to intimately attach did not affect the level of E. coli O157:H7 adherence to bovine intestinal epithelium in vitro. A strain lacking the genes required for intimate attachment however exhibited enhanced adherence to bovine Peyer’s patch. The other strains did not exhibit tropism for any of the tissue type examined. The most relevant system to assess the behaviour of E. coli O157:H7 is within its natural host. Persistent colonisation of weaned calves was achieved for a number of isolates marked by nalidixic acid resistance, including a Stx negative strain that colonised at a similar level and duration to the equivalent Stx positive strain. At the conclusion of each calf colonisation experiment, those individuals still shedding the organism were examined under necropsy to determine its distribution. The first attempts failed to recover the organism in significant numbers at any site examined despite it being present in ante-mortem faeces. One explanation was that the organism was multiplying primarily in the distal rectum. Further necropsies revealed that the organism was colonising the mucosal surface of the distal 3 cm of the rectum via intimate attachment and confirmed that this phenomenon was indeed typical of persistently colonised calves. This small region contained a high density of lymphoid tissue.