A carcinoid tumor was found as a solitary soft mass in the wall of the rectum adjacent to the anorectal junction in an adult Holstein cow. Microscopically, the tumor involved the submucosa and partly invaded the muscular layer. It consisted of a compact arrangement of a great number of large polygonal cells and a small number of small dark cells, some of which showed argyrophilia (Grimelius positive). Immunohistochemically, both types of tumor cells were positive for vimentin, keratin, and S-100 protein and weakly positive for neuron-specific enolase (NSE), whereas they were negative for some endocrine markers such as chromogranin A, insulin, glucagon, somatostatin, serotonin, adrenocorticotropic hormone, and calcitonin. Electron microscopy revealed membrane-bound secretory granules in the cytoplasm of some small dark cells. In such a poorly differentiated carcinoid, the morphologic characteristics of the small dark cells were strong evidence for the diagnosis. This is the first description of a poorly differentiated carcinoid developing in the rectum of a cow.