Abstract: Drum reactor (10) for aerobic fermentation has a frame (12) with which a height adjusting device (14). The drum reactor is provided with a rotary drum (11) having air pipes (20) extending longitudinally along its outer shell. The air pipes (20) are subject to compressed air and humidity by the action of a distributor slide valve (25) so that the air pipe (37) which is free of fermenting material (26) is inactive. Associated with the air pipes (20) are a plurality of jets (21) which lead into the drum interior (27) where they are at least partially covered by baffles (22). By their plough-like disposition, the baffles ensure circulation and onwards conveyance of the material to be fermented. Provided on the drum bottom (38) is a discharge orifice (33) which can be closed by a plough flap (36). For delivery of fermented material, the plough flap can be spread open into the drum interior.