Chemical and ultrastructural characteristics of rice straw degradation by ruminal cellulolytic bacterium, Ruminococcus albus F-40, were investigated. The degadation of rice straw and the adhesion rate of bacteria to the substrates increased with the decrease of cellulose crystallinity, suggesting the preferential degradation of amorphorous cellulose by R. albus. Polarized light microscopy and X-ray diffraction analyses, however, showed that crystalline cellulose in rice straw was also degraded by R. albus FT-IR spectra indicated that hemicellulose was also degraded by this bacterium. Electron microscopic(EM) investigations showed the protuberant structures on the surface of R. albus. Glycocalyx extended from the bacterial surface adhered tightly to the cellulosic substrates and vesicles occurred to the surface of cellulose substrates under the degradation. Cellulosome-like structures occurred on the surface of bacterial cells and on the cellulose microfibrils as well. EM examinations revealed that the degradation of cellulose by R. albus was proceeded as 1) adhesion of bacterium on the substrates 2) hemicellulose degradation 3) cut-off of axial fiber cellulose and 4) defibrillation of cellulose microfibril. Immuno-EM studies using the polyclonal antibodies of endocellulase from bacterial cell surface showed the intense gold labelling on the cellulose of rice straw at the beginning stage of degradation. Positive gold labelling in vesicles and glycocalyx indicated that these surface structures were involved in the degradation of cellulose. Catalytic domain of endocellulase was localized externally along the axis of cellulose microfibrils, indicating the direct involvement of this enzyme fraction in the degradation of cellulose.