Strictly anaerobic fungus species, present in the digestive tract of herbivores, are widely distributed. as confirmed by the discovery in France of Piromyces mae Li in sheep rumen. By the study of a new strain, the morphological characteristics of this species were more accurately determined and the presence of megahydrogenosomes in its zoospores confirmed. Chitobiose. N-acetylgalactosamine, and arabinose were demonstrated on the thallic surface with lectins of different affinities for the sporangium or the rhizoids. In contrast to P. communis, there were no surface oligosaccharides with a terminal galactose bound to N-acetylgalactosamine or N-acetylglucosamine by a beta 1 --> 3 bond.