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An ulcerated and pedunculated intraluminal yellowish solitary mass was observed protruding into the ruminal lumen of an adult cow during an abattoir survey. Histologically, the neoplasm invaded the lamina propria-submucosa, eroded the ruminal epithelium and segmentally effaced the inner tunica muscularis. It was composed of pleomorphic spindle cells arranged in fascicles. Areas of hemorrhage, necrosis, microcystic changes as well as marked anisokaryosis, the presence of giant cells and scattered mitosis with atypical figures, were also observed. Immunohistochemically this tumor labeled positive for alpha smooth muscle actin, desmin and vimentin. With all the above findings, a diagnosis of ruminal leiomyosarcoma was confirmed. To the authors' knowledge, this is the first report of ruminal leiomyosarcoma in cattle.