Platypuses (Ornithorhynchus anatinus) in the north of the island state of Tasmania, Australia, suffer from a serious disease called ulcerative mycosis, which is responsible for high morbidity and, presumably, mortality rates in areas where it occurs. The disease is caused by the dimorphic fungus Mucor amphibiorum, which is also found in Queensland, New South Wales and Victoria. However, it does not cause disease in platypuses in those states. It has been previously reported that a closely related fungus, Mucor circinelloides, may also be capable of causing this disease. This paper describes pathogenicity trials involving cane toads (Bufo marinus) as the experimental model. The toads were infected with either Tasmanian, platypus-derived M. amphibiorum, West Australian, frog-derived M. amphibiorum, Queensland cane-toad-derived M. amphibiorum or Tasmanian platypus-derived M. circinelloides. The Tasmanian isolates of M. amphibiorum were more likely to cause a serious, long-term infection than were Queensland or West Australian isolates, and (+) mating types caused a more serious infection than the (-) mating type. The isolate of M. circinelloides was incapable of infecting the toads, lending further weight to the theory that it represents an environmental contaminant. The results suggest that an endemic strain of M. amphibiorum has mutated and become pathogenic to platypuses. Alternatively, a pathogenic strain of M. amphibiorum may have been introduced into Tasmania, where it is infecting a naïve population.