Twenty-six gastrointestinal tumors were observed in twenty-three nonhuman primates during routine necropsies at the German Primate Center, Göttingen. The majority (15 cases) were colorectal mucoid adenocarcinomas in cotton-top tamarins (Saguinus oedipus), which in two animals were associated with gastric adenomas. Three cases of small intestinal mucoid adenocarcinomas occurred in common marmosets (Callithrix jacchus). One colonic leiomyoma was observed in a dwarf galago (Galagoides demidovii) and another one in a cotton top tamarin. Singular findings were a tubular adeno-carcinoma of the ileo-caecal valve in a saddle-backed tamarin (Saguinus fuscicollis) and a lymphosarcoma of jejunum, ileum, and colon in another saddle-backed tamarin. Multiple tubular adeno-carcinomas of the colonic diverticles occurred in an aged rhesus monkey (Macaca mulatta). The findings are discussed in comparison to the situation in man.