Allergic dermatitis was studied at a farm located in the municipality of Castanhal in the state of Pará, Brazil, from December 2009 to December 2012. Clinical and epidemiological data were obtained during monthly visits to the farm. Skin biopsies were performed for histopathological examinations, and insects were captured with Centers for Disease Control and Prevention (CDC) traps and live-bait entomological vacuum devices. The property housed a herd of both genders, with 45 Santa Inês sheep of different ages and 72 Texel adults. Of the 72 Texel sheep, 56 were affected. Of these, 24 exhibited alopecia, erythema, edema, and crust in the ears, on top of the head, and less often, around the eyes. In addition to the lesions described above, 14 animals exhibited hyperpigmentation of the ears and/or around the eyes and crust in the nose. Another 18 animals exhibited chronic lesions characterized by deformed and thickened ears, alopecia with hyperpigmentation, and in most cases, secondary bacterial infections with abscesses and/or myiasis. Some of these animals had lost part of their ears. Histologically, the lesions were characterized as orthokeratotic hyperkeratosis, acanthosis, vacuolization, and necrosis of epidermal cells. An inflammatory infiltrate was present in the dermis, which was composed mainly of eosinophils and macrophages, some lymphocytes and mast cells, and few neutrophils, in addition to edema and collagen deposition. The Santa Inês sheep were not affected. Among the captured insects, 294 specimens of Culicoides (Hoffmania) plaumanni Spinelli, 57 specimens of C. (Hoffmania) insignis Lutz, and 27 specimens of other Culicoides species were identified. We conclude that C. plaumanni, C. insignis, and possibly other Culicoides species are involved in the etiology of allergic dermatitis in Texel sheep in the state of Pará. Santa Inês sheep in the same region and under the same conditions were not affected, which suggests lower susceptibility of these animals.