AbstractA novel two‐stage anaerobic process for the microbial conversion of cellulose into biogas has been developed. In the first phase, a mixed population of rumen bacteria and ciliates was used in the hydrolysis and fermentation of cellulose. The volatile fatty acids (VFA) produced in this acidogenic reactor were subsequently converted into biogas in a UASB‐type methanogenic reactor.A stepwise increase of the loading rate from 11.9 to 25.8 g volatile solids/L reactor volume/day (g VS/L/day) did not affect the degradation efficiency in the acidogenic reactor, whereas the methanogenic reactor appeared to be overloaded at the highest loading rate. Cellulose digestion was almost complete at all loading rates applied. The two‐stage anaerobic process was also tested with a closed fluid circuit. In this instance total methane production was 0.438 L CH4g VS added, which is equivalent to 98% of the theoretical value. The application of rumen microorganisms in combination with a high‐rate methane reactor is proposed as a means of efficient anaerobic degradation of cellulosic residues to methane. Because this newly developed two‐phase system is based on processes and microorganisms from the ruminant, it will be referred to as “Rumen Derived Anaerobic Digestion” (RUDAD‐) process.