Anaerobic fungi were first detected in faeces produced by newborn calves and lambs at 4 and 5 wk after birth, respectively. Populations developed gradually thereafter, until eventually the faeces produced by all young ruminants contained anaerobic fungi. Anaerobic fungi were detected in the rumen of calves prior to their detection in faeces. Rumen digesta and faeces of adult Friesian dairy cows contained larger populations of anaerobic fungi than those detected in young ruminants. Introduction of wheat straw particles, in small polyester bags, into the duodenum of fistulated adult cattle and retrieval of these bags from voided faeces established that the hindgut contained an active population of anaerobic fungi. In faeces, anaerobic fungi were similar to the monocentric and polycentric fungi commonly found in the rumen. The ability of faecal isolates to degrade plant cell-wall polymers was comparable to that of the rumen fungus, Neocallimastix hurleyensis. However, whereas fungal populations in faeces were able to withstand drying in air at ambient temperature, those present in rumen digesta failed to survive the drying process. These differences in the behaviour of anaerobic fungi in rumen digesta and faeces suggest that the faecal fungi are able to persist and survive due to the formation of cysts or resistant zoosporangia.