The distribution and number of Peyer's patches (5) in two species of marsupial mice, Antechinus swainsonii and Antechinus stuartii was found to be the same even though the length of the intestine in the latter species was half that of the former. Both species lack a caecum and appendix. The position of the Peyer's patches is unusual in that the first three Peyer's patches are on the right side of the small intestine whereas the penultimate and ultimate Peyer's patches are large, contain many lymphoid follicles and are in an anti-mesenteric position in the small intestine and sometimes in the large intestine (ultimate Peyer's patch). The number of Peyer's patches in eutherian mice of similar size, and intestinal length is greater (13) although the number of Peyer's patch lymphoid follicles per centimeter intestine is less (1.8) than in A. swainsonii (4) and A. stuartii (4.4). Marsupial mice have most of their lymphoid follicles confined to a few large Peyer's patches, whereas eutherian mice have fewer lymphoid follicles per unit intestinal length, more Peyer's patches (with fewer lymphoid follicles) evenly distributed along the intestine and more single lymphoid follicles interspersed between them.