The cellulosic substrate in a compressed state is introduced in successive batches into the feed zone (15) of a fermenter (2); a quantity of the substrate which is suitable for anaerobic fermentation is introduced continuously into the zone (15). The working mass travels in the direction of the extraction zone (17) of the fermenter, from which the two degraded substrates are extracted. The biogases formed are collected continuously at the top of the fermentation zone (16). The originality of the process is that it subjects each of the batches of the cellulosic substrate, as it enters the zone (16) to an additional compression, which results in an increase in its density, the consequence of this being an increase in the bacterial fixation potential. The substrate which is suitable for fermentation can be recycled without any pumping means; the compression of a batch of cellulosic substrate in fact causes a rise in the level of the substrate which is suitable for fermentation in the zone (17) at the same time as a lowering in this level in the zone (15).