Mucor amphibiorum, a fungus previously isolated from frogs and toads, is reported from free-living platypus, Ornithorhynchus anatinus, from rivers in northern Tasmania. This fungus is responsible for the severe ulcerative skin condition originally described by Munday and Peel (1983). Mucor amphibiorum was isolated from dermal lesions on four separate occasions. The gross and histopathological appearance of the fungal lesions were similar to the earlier description. In vivo this fungus develops as spherical forms containing a number of daughter spherules; no mycelia are seen in tissue sections. By contrast, the in vitro growth consists of aerial aseptate mycelia and sporangia, features typical of the genus Mucor. This is the first report of this organism causing a fatal disease in a mammal. Susceptibility to infection may be due to the platypus having a body temperature of 32 C while the maximum temperature for growth of M. amphibiorum is 36 C.