NOVELTY - Fermenter (1) comprises a biogas plant (10) with at least one fermenter wall (2), at least one fermenter interior (3), at least one fermenter roof (5), which is flexibly formed and inclined to the horizontal, and at least one stirring device for stirring a substrate present in the fermenter interior. The stirring device comprises at least one guide unit, at least one support unit and at least one stirring unit, which is adjustably placed at the guide unit. The support unit for supporting the guide unit is fixed to the fermenter wall. The stirring unit is vertically adjustable.    USE - Used as a fermenter.    ADVANTAGE - The fermenter: is produced in a cost efficient manner; is easy to maintain; is easy and simple to operate.    DETAILED DESCRIPTION - Fermenter (1) comprises a biogas plant (10) with at least one fermenter wall (2), at least one fermenter interior (3), at least one fermenter roof (5), which is flexibly formed and inclined to the horizontal, and at least one stirring device for stirring a substrate present in the fermenter interior. The stirring device comprises at least one guide unit, at least one support unit and at least one stirring unit, which is adjustably placed at the guide unit. The support unit for supporting the guide unit is fixed to the fermenter wall. The stirring unit is vertically adjustable. At least one adjusting device is provided for adjustment of the stirring unit. An upper end of the guide unit extends above an upper end of fermenter wall and ends below the fermenter roof within the fermenter interior. An INDEPENDENT CLAIM is also included for the stirring device, preferably for the fermenter of the biogas plant for stirring the substrate present in the fermenter interior, comprising at least one guide unit and at least one support unit.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic side view of the fermenter.    Fermenter (1)    Fermenter wall (2)    Fermenter interior (3)    Fermenter roof (5)    Biogas plant (10)