Thirty-four Mystromys albicaudatus were injected intradermally with Leishmania braziliensis promastigotes and examined and killed during a 12-week period. All animals developed single cutaneous lesions at the sites of inoculation that began as a papular thickening in the dermis and progressed to a 2.0 cm crateriform ulcer. The histopathology of these lesions was characterized by granuloma formation and diffuse infiltrates of lymphocytes and plasma cells. Parasites were most frequently observed in vacuolated macrophages immediately underlying the necrotic debris of the ulcer. Histiocytes in which amastigotes were not identified were noted in aggregates at the margins of the inflammation. Russell's bodies were observed at 6 weeks post-infection, and discrete lymphocytic infiltrates bordered the inflammation at 12 weeks post-infection. It is suggested that M. albicaudatus is an excellent model of American cutaneous leishmaniasis.