Ulcerative dermatitis observed in several shipments of rats was found to be limited to one strain (Sprague Dawley) provided by one supplier from two specific breeding areas. The incidence of lesions was higher in males than in females. Rats receiving an agar base diet also appeared to have a higher incidence of the lesions. The lesion consisted of a moist ulcer usually involving the lateral aspect of the shoulder and often the sides of the neck. Staphylococcus aureus was isolated from lesions of affected rats and from the skin of unaffected rats. Inoculation of S aureus into susceptible animals resulted in lesions in 25-40% of the rats, but usually at sites other than the site of inoculation. Serum neutralization tests for sialodacryoadenitis virus indicated that about half the rats with lesions and half the rats without lesions were positive. Toenail clipping of the hind feet of affected rats reduced the severity of the lesions. Efforts to isolate fungi and ectoparasites were unsuccessful. The selective procurement of rats from specified breeding areas, eliminated the population susceptible to or harboring the etiologic agent or agents responsible for the development of the ulcerative dermatitis described.