Effects of packing of the filter media on substrate degradation and on behavior of anaerobic bacteria were investigated using an upflow anaerobic fixed bed reactor operated at 20°C. A low strength synthetic wastewater containing suspended solids was used as the substrate. Here, cellulose corresponds to suspended solids in domestic sewage. The effluent water quality is improved remarkably by packing of the filter media. Packing of the filter media promotes the degradation of insoluble matter as well as soluble matter of the influent. Insoluble matter (cellulose) in the influent does not accumulate in the interstitial space of the filter media and on the surface of the filter media, but acclimates in the lower part of the reactor. Packing of the filter media promotes the accumulation of lipolytic bacteria, acetate consuming methanogenic bacteria and hydrogen consuming methanogenic bacteria in the space where filter media were packed. Hydrolysis reaction of cellulose is promoted by packing of the filter media.