Abstract: An anaerobic production of acetic acid by fermentation can employ homoacetogenic bacteria in an aqueous solution of fermentable substrates, especially sugar, with the result of increased acetic-acid concentrations (over 3%), when the substrate solution initially used in the fermentation already contains acetic acid, for example, from a previous fermentation. In a fed-batch process, acetic acid is preferably present in a concentration ranging from about 0.5% to about 2%, and especially around 1%. At least two fermenters in compound (interconnected) arrangement are used. A real continuous fermentation is realized in at least two sequentially-operated fermenters by regulating the residence time in the first at about 1 day and in the second fermenter at at least 2 days.