Seasonal allergic dermatitis is reported in a lock of 40 hair sheep of the Santa Inês breed, in the county of Jucurutú, State of Rio Grande do Norte, in the semiarid region of Northeastern Brazil. For the epidemiologic and clinical observations the farm was visited periodically from 2007 to 2010. The lock was raised extensively in a Paddock crossed by the Piranhas river, and had also a permanent pond. Between 2007 and 2009, 13 (32.5%) sheep out of 40 were affected. The lesions were observed during the raining season, regressed total or partially during the dry season, and reappeared in the next raining period. Sheep removed to other region recovered. At the end of 2009, all affected sheep were removed from the lock. Three new cases appeared in 2010. The skin of the affected sheep was whitish, irregularly thickened, with alopecia, crusts, and intense pruritus. Those lesions were localized more frequently in the head (ears and periocular and frontal regions) but in some animals the dorsum and croup were also affected. Histologic lesions of the epidermis were hyperkeratosis, acanthosis, hypergranulosis, and moderate spongiosis. The dermis showed in iltration by eosinophils, macrophages and plasma cells, mainly perivascular. The hair follicles showed keratosis and the sudoriparous glands were dilated. No changes were observed in the differential blood count. In April and June 2010 insects were captured by the use of CDC light trap. Forty three out of 110 dipterous captured were identi ied as Culicoides insignis Lutz. It is concluded that the disease is a seasonal allergic dermatitis associated with C. insignis bites.