The effects of Aspergillus oryzae fermentation extract, AmafermTM, on the rumen fungus Neocallimastix frontalis EB188 were studied. The secretion of cellulase was increased by 67% and rhyzoid development was increased 3·8-fold in the presence of extract. Strength of fungal response increased in a dose-dependent manner and demonstrated a positive correlation between cell surface area and enzyme secretion. Above certain concentrations of extract, however, the development of the fungus and enzyme secretions remained at control values or slightly diminished. Supernatant fluid appearance of the intracellular enzyme, malate dehydrogenase, paralleled the secretion of cellulase both in the presence and absence of extract. Ether solubilization of extract demonstrated that the active component(s) possessed a moderately polar value between 2·7 and 2·8. Thin layer chromatography separated extract into inert, inhibitory and intensely stimulating fractions. These results support the idea that by accelerating fungal growth and metabolism, AmafermTM increases the rate (or extent) of fibre degradation caused by rumen fungi and that this, in turn, may contribute to enhanced animal performance.