The diet of the long-nosed potoroo, Potorous tridactylus, in south-western Victoria, was investigated by the microscopical identification of faecal remains. P. tridactylus is omnivorous: the main component of the diet was fungi, and other important items included hard-bodied arthropods, vascular plant tissues, seeds and fleshy fruits. There was a seasonal switch in the relative proportions of the main dietary components between the autumn-winter and spring-summer periods of the year. During autumn and winter, the main components were fungi and seeds. In spring and summer, fewer fungi were eaten and the proportions of arthropods, plant tissues, fleshy fruit and flowers in the diet increased. Identification of fungal spores revealed the presence of at least 50 species in the diet, most of which have a hypogeal fruiting habit. Hypogeal fungi form ectotrophic mycorrhizal associations with forest trees and are important in the health and productivity of forests. They lack active mechanisms for spore dispersal and are dependent upon mycophagous animals. The role of mycophagous small mammals, such as P. tridactylus, in the health of forest ecosystems may be more important than previously recognised.