Congenitally athymic (nu/nu) mice, mice defective in NK cell and macrophage function (bg/bg), and normal BALB/c mice were inoculated sc with 105–6 conidia of Fonsecaea pedrosoi (FP). In immunologically intact and immunodeficient mice, a local infection developed approximately 2 weeks post-inoculation and enlarged over 1–2 weeks. In bg/bg and normal nu/+ mice, lesions resolved within 5–6 weeks. However, nu/nu mice continued to have enlarging sc lesions during >4–6 months of observation. These eventually metastasized. Lesions contained few hyphal elements and massive numbers of sclerotic bodies. Five weeks after inoculation, 104–6 conidia forming units/gm of tissue were recovered from lesions. Delayed type hypersensitivity and serum antibody to FP antigens were demonstrated. Adoptive transfer of lymphocytes from nu/+ mice was followed in 2 months by the resolution of the lesions.