Abstract: For the fermentative preparation of polysaccharides, in particular xanthane, water, production medium (sugars, nutrient salts), and an inoculation material of a suitable aerobic bacterium are mixed in a reactor as reaction components, the production medium being exposed to fermentation while an oxygen-containing gas is being supplied. The object is to avoid the usual low yield of such a process and to reject an expensive and sensitive alternative construction (circulation tube). According to the invention, the reaction components are introduced into a reactor vessel in a circulating flow by means of pumping means, which circulating flow comprises a rising flow and a descending flow which are separated from each other by one or more partitions disposed in the reactor vessel. The mixing is brought about because the rising flow and/or the descending flow is passed essentially in the form of a plug flow through one or more static in-line mixers.