Studies on fiber digestion from plant materials in ruminants have placed emphasis on the process of degradation in the rumen. Cellulose appears to be essentially 100% degradable in the rumen irrespective of feeding regimen, provided it is in the pure state. Degradation of fiber fractions from plant materials in the rumen decreased to 40 to 50%, presumably due to the presence of pentoses such as xylose and arabinose in these materials. Particle size reduction associated with the ruminal retention time of feed particles may be involved in the degree of fiber degradation in the rumen, though this factor has been found to be of minor importance. Degradation of fiber fractions from plant materials occurs also in the post-ruminal digestive tract. The cecum contributed about 20% of fiber digestion in the gastrointestinal tract and the contribution of the rumen was estimated to be about 80%. The pattern of mass base frequency of digesta particles was found to be similar in the omasal and rectal contents and no significant difference was observed in the average diameter of the particles in the omasal and rectal contents. The pattern of particle distribution in feces may provide useful information for predicting the degradation of fibrous fractions from plant materials in the rumen.