The anatomy of the cecum of the domesticated guinea pig (Cavia aperea f. procellus) was investigated at the macroscopic and microscopic levels. In situ observations, injections of the blood vascular system and the preparation of dried specimens were made to elucidate the macroscopic anatomy of the cecum. The complex mesenterial situation in the abdomen was also investigated. The guinea-pig intestinal tract is peculiarized by the presence of the voluminous teniated cecum, which lacks an appendix vermiformis. The stomach is relative small and simple; the large intestine does not possess teniae. Seen with the scanning electron microscope the surface topography of the cecal wall shows a pattern of irregular mounds and crevasses. The former are made up of raised ridges, often in circular or looped profiles. Correlated light microscopic observation revealed an irregularity of the mucosal surface consisting of protrusions into the cecal lumen, the circular and looped configurations of the scanning microscope image representing the entry into crypts at the light microscopic level. The close association of bacteria to the mucosal surface is striking. Observed with the transmission electron microscope this close association takes the form of an intermicrovillar location of the bacteria. The present observations are compared to those made on their rodents and with respect to the dietary habit of the guinea pig.