The growth characteristics of 17 isolates of anaerobic fungi from various animal hosts were investigated relative to their ability to degrade wheat straw. The isolates consisted of 11 Neocallimastix isolates from Malaysia, twoPiromyces isolates from Australia and Neocallimastix hurleyensis, two polycentric isolates and aCaecomyces isolate, all from the U.K. These fungi were grown on wheat straw in batch culture and their characteristics determined from the wheat straw apparent dry matter (DM) loss data and a mathematical description of their fermentation gas production profiles as measured using a pressure transducer. On the basis of the apparent DM loss data, isolates were divided into three groups, viz., good (Malaysian and Australian isolates), moderate (U.K. isolates) and poor (U.K. and Australian isolates) degraders of wheat straw. From their gas production profiles, the majority of the Neocallimastix isolates had similar lag times, specific growth rates and fermentation gas pool sizes. However, when isolates were grouped according to their geographical location, the gas pool size and final DM loss for the U.K. isolate, N. hurleyensis, was significantly lower than those of the Malaysian Neocallimastix isolates. The apparent DM loss and gas production data for all isolates were subjected to multivariance analysis and this confirmed similarity between the Malaysian isolates. The resulting hierarchical cluster tree separated the good degraders into two distinct groups consisting of the Malaysian and Australian isolates. This research confirms that the gut fungi exhibit isolate-dependent differences in their activity towards recalcitrant substrates like wheat straw. The work also demonstrates the precision of the pressure transducer and associated modelling procedures in determining the kinetics of gut fungal growth on particulate substrates, enabling distinctions to be made between apparently similar fungal isolates.