The rumen microbiome constitutes a unique genetic resource of plant fiber degrading microbial enzymes that could be used for agricultural and industrial purposes. Anaeromyces mucronatus is a poorly characterized anaerobic lignocellulolytic fungus in the rumen. This thesis aimed at better understanding A. mucronatus YE505 and the particle associated rumen microbiota based on transcriptomic and metatranscriptomic approaches. High quality RNA was isolated from the fiber-associated rumen sample based on an improved RNA extraction method. A transcriptomic study was performed to investigate the expression of the fiber degrading system of A. mucronatus YE505, and the functional diversity of the fiber-associated eukaryotes from the rumen of muskoxen (Ovibos moschatus) was explored by a metatranscriptomic study. Much carbohydrate degradation related protein modules were detected. This study established effective approaches to characterizing the functional contents of rumen eukaryotic microbiome as well as rumen fungi, and identified several candidate genes that merit further investigation. / xiv leaves : ill. (some col.) ; 29 cm