In order to clarify the fundamental structure of the host defence mechanism in chicken caeca, a detailed analysis of the distribution of lymphoid nodules (LNs) was carried out on longitudinal sections of both the mesenteric (side of the ileocaecal ligament) and the antimesenteric mucosa. An overwhelming majority of solitary or aggregated LNs were located in the mesenteric mucosa, although a few were also found in the antimesenteric mucosa. Of the total LNs, 45.7% were detected at the proximal 7.8% section in the caecal tonsil. LNs (21.4%) were also concentrated in the distal 22.0% section corresponding to the apex. A moderate concentration of LNs (13.1%) was found at the transitional 20.0% region between the base and body. Approximately 80.2% of total LNs were found at the above 3 regions in the mesenteric mucosa. In many cases, the frequency of LNs in the caecal tonsils was opposite to that at the apices. Aggregated LNs were mainly found in the caecal tonsils, transitional region and apex. Almost all aggregated LNs consisted of fundamental nodular units possessing M cells in their follicle associated epithelia. The aggregated LNs in the above 3 regions therefore could provide immunological surveillance against caecal luminal contents. In particular, the cooperative function between LNs of the caecal tonsil and apex might be highly important in maintaining the caecal microenvironment.