Abstract: An apparatus for cultivating immobilized micro-organisms consisting of a fluidized bed reactor with fittings for gassing and a retainer system for the immobilized biocatalysts. The fluidized bed reactor is characterized in that the fittings for gassing consist of several segments with gas outlets arranged on the base of the fluidized bed reactor, which admit gas alternately or cyclically. A central gas outflow opening, pointing upwards, can be provided in the fluidized bed reactor as a further gassing unit. The method of operating the fluidized bed is characterized in that circulating gassing occurs in the reactor by means of sequential opening of the gas inflow openings of the segment-shaped fittings. In addition, switches may occur from the central gassing unit to gassing by means of the segment-shaped fittings or vice-versa.