Mammary tumors of all types are rare in herbivores, and there is a particular paucity of reports in sheep. The present report describes a case of mammary carcinoma in a 6-year-old uniparous ewe. The tumor was present for at least 18 months, during which time the ewe remained clinically well. At postmortem examination, the mass was found to be multilobulated with occasional cysts. Histologically, the lobules consisted of tubules lined by cuboidal to low columnar epithelium with loss of polarity and moderate anisokaryosis within a moderately extensive fibrous stroma. It was classified as a low-grade carcinoma. The histologic classification and lack of evidence of invasion correlated well with the slow clinical progression.