The ability of five ruminal fungi in syntrophic co-culture with the methanogen Methanobrevibacter smithii to degrade perennial ryegrass (Lolium perenne) stem fragments and leaf blades was studied to determine the susceptibilities of non-autoclaved fresh tissues to fungal degradation, Autoclaving did not significantly increase fungal degradation of stem fragments but strongly increased degradation of leaf blades by a species of Caecomyces. In methanogenic co-cultures. non-autoclaved stem fragments were degraded more extensively by Neocallimastix frontalis and Piromyces isolates than by Caecomyces isolates, The N. frontalis and Piromyces isolates showed the greatest rates of stem degradation. When interactions between Fibrobacter succinogenes and methanogenic co-cultures of fungi growing on ryegrass stem were investigated, N. frontalis inhibited F. succinogenes. This has not been observed previously. In contrast, a Caecomyces species interacted positively with F, succinogenes to increase stern degradation, suggesting that F. succinogenes and Caecomyces spp. may have complementary fibrolytic activities. All five fungi tested failed to grow on fresh non-autoclaved leaf blades. In I repeat experiment with leaves from a separate harvest, leaf blades were degraded by N. frontalis but not by a Caecomyces species. We suggest that ryegrass leaf blades may contain natural anti-fungal compounds. Our results confirm the superiority of fungi in the degradation of intact stem and indicate that in vitro studies with non-autoclaved forage tissues may yield new information on forage factors affecting rumen microbes.