NOVELTY - Agitator frame (12) comprises at least one bearing mount formed on the agitator frame for receiving an upper agitator shaft bearing to vertically mounting an agitator shaft, at least one drive mount associated with the bearing mount and formed on the agitator frame for receiving an agitator drive for the agitator shaft, and an anchoring device for anchoring the agitator frame relative to a fermenter body of a fermenter (1) such that the agitator frame, the agitator shaft and the agitator drive are completely receivable within the fermenter body.    USE - The agitator frame is useful for agitator arrangement of fermenter to produce biogas (all claimed).    ADVANTAGE - The agitator frame avoids roof opening and its sealing problem, reduces leakage, and ensures sealing and maintenance of the fermenter in simple manner. The biogas plant is safe and economical.    DETAILED DESCRIPTION - Agitator frame (12) comprises at least one bearing mount formed on the agitator frame for receiving an upper agitator shaft bearing to vertically mounting an agitator shaft, at least one drive mount associated with the bearing mount and formed on the agitator frame for receiving an agitator drive for the agitator shaft, and an anchoring device for anchoring the agitator frame relative to a fermenter body of a fermenter (1) such that the agitator frame, the agitator shaft and the agitator drive are completely receivable within the fermenter body. The agitator drive and the upper agitator shaft are arranged above a maximum substrate filling height. INDEPENDENT CLAIMS are also included for:    (1) an agitator arrangement, comprising at least one agitator having the agitator shaft, the agitator drive for driving the agitator shaft, and the bearing mount for axially and radially mounting the agitator shaft, and the agitator frame for receiving the agitator shaft, the agitator drive and at least one portion of the bearing mount of the agitator;    (2) fermenter for producing biogas, comprising the fermenter body, a substrate introducing device for introducing substrate to be fermented into interior of the fermenter body, a substrate extraction device for removing fermented substrate from the fermenter, and the agitator arrangement for agitating the substrate, where the agitator drive is arranged within the fermenter body, preferably above maximum substrate filling height of the fermenter;    (3) a biogas plant comprising the above fermenter;    (4) producing biogas, comprising fermenting the substrate in the fermenter, where the substrate is mixed at least partially in laminar manner; and    (5) converting the fermenter to produce biogas, comprising installing the agitator frame in the fermenter body of the fermenter.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the fermenter with the agitator frame.    Fermenter (1)    Base plate (4)    Side wall (5)    Roof (6)    Agitator frame (12)