Ileal Peyer's patches of rhesus macaques were investigated by light and electron microscopy (scanning-electron microscopy, transmission-electron microscopy). The results were compared with findings in other species. Differences were found concerning the shape of the dome area, the composition of the dome epithelium and the apical membrane of M cells: in the rhesus monkey, hemispherical domes bulge into the intestinal lumen. The dome epithelium is composed of three populations of gut epithelial cells; absorptive enterocytes as the predominant cell type; goblet cells; and M cells ("microfold bearing' or "membraneous' gut epithelial cells). The apical membrane of M cells forms irregular protrusions.