Severe dermatomycosis (ringworm) caused by an unidentified dermatophyte occurred in a mature, debilitated, female mule deer (Odocoileus hemionus) from southwestern Alberta. Lesions involved much of the body surface and were characterized by severe alopecia of the face, lower thoracic wall and abdomen, perineum and limbs. The skin was markedly encrusted and scaly in all areas. The histologic lesions included marked hyperkeratosis and a chronic dermatitis with the presence of numerous spherical ecto- and endothrix arthrospores and segmented mycelial elements. The causative organism could not be grown on artificial media, but the distribution and morphology of arthrospores, the presence of segmented mycelia and the nature of the inflammatory reaction, suggested infection by a Trichophyton species. This is the first report of dermatomycosis in a free-ranging big game animal in North America.