The invention relates to a device for fermenting organic substances, especially a fermenter or decomposition container (1; 101). Said fermenter or decomposition container comprises a fermenter chamber (2; 102) that has a substantially vertically aligned longitudinal axis and that receives the organic substances up to a defined level (2a). The fermenter chamber further comprises a mixer device comprising driven mixer elements (3a to 3g; 103; 203; 303; 403; 503). Said mixer elements (3a to 3g; 103; 203; 303; 403; 503) can be moved out from the organic substances so that they project at least partially beyond the level (2a) and are then at least partially dipped back into the organic substances so that a layer or deposit floating near or on the surface can be torn open.