Nonpredisposed Albino guinea pigs were infected intravenously with Rhizopus microsporus var. rhizopodiformis or with Rhizopus oryzae. Both strains were highly pathogenic. They killed all control animals between days 4 and 7 and between days 5 and 9 after infection, respectively. All animals presented invasion of almost all internal organs and skin eruptions developing into ulcers. Oral treatment with ketoconazole, itraconazole, fluconazole or saperconazole was inefficacious. Parenteral treatment with amphotericin B prolonged survival and was life-saving in 9 out of 12 guinea pigs infected with Rh. microsporus var. rhizopodiformis and in 5 out of 12 infected with Rh. oryzae. More active therapy is needed.