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 ketocon-azole, 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.