A solid and firm tumor, covering the whole left ovary was detected in 10 years old a troughbred mare, which had a 2-year history of infertility. The cut surface of the mass was whitish-yellow with whorled appearance. Histologically, tumor was composed of uniform spindle cells arranged intersecting fascicles. Intercellular kollagen, which was stained blue with Mallory's triple was observed in stroma. Focally, the spindle cells showed abundant vacuolated cytoplasm, which was a feature for thecal differentiation. Neither sex cord islands nor granulosa cells were observed in that tumor. The mitotic amount averaged less than 2 cells. Immunohistochemically, the spindle-shaped neoplastic cells were positive for vimentin, whereas only weak tram-track immunoreactivity was demonstrable for desmin and smooth muscle actin. Based on the morphological and histological features, the tumor was classified as a fibrothecoma.