The ruminal fungi Neocallimastix frontalis RE1, Neocallimastix patriciarum 27, Piromyces communis 22, and Orpinomyces joyonii 19-2 were examined for their ability to digest filter paper in the presence of condensed tannins from birdsfoot trefoil (Lotus corniculatus L.). For all four fungi, inhibition of endoglucanases was evident at 100 micrograms condensed tannins.mL-1 with nearly complete inhibition at 300 micrograms condensed tannins.mL-1. At 100 and 200 micrograms condensed tannins.mL-1, the endoglucanase activity of N. frontalis RE1 was greater (P < 0.01) than that of the other three fungal species. Exposure to 100 micrograms condensed tannins.mL-1 did not affect the ability of N. frontalis RE1 or N. patriciarum 27 to digest filter paper, and although digestion was reduced, N. frontalis RE1 and P. communis 22 solubilized more than 20% of the filter paper at 500 micrograms condensed tannins.mL-1. In contrast, O. joyonii 19-2 was virtually unable to digest filter paper at 300 micrograms condensed tannins.L-1. Mycelia of fungi grown with condensed tannins were covered by filamentous material, which may have arisen from the formation of condensed tannin-protein complexes. Less than 86% of the condensed tannins (as measured by the H2SO4 method) were recovered after 120 h of incubation with N. frontalis RE1, P. communis 22, and N. patriciarum 27. The need for detailed studies to examine the ability of ruminal fungi to metabolize condensed tannins is evident.