A fedbatch strategy was developed coupling the feeding of the two inhibitory substrates glycerol and ammonium to alkali consumption. A continuous, automated substrate addition was achieved responding directly to the needs of the culture. Thus substrate concentrations were kept on a constant low, but non limiting level. The feeding was applied for the cultivation of Clostridium butyricum DSM 5431 and mutants with increased product tolerance. Compared to fedbatch cultivations with intermittent feeding cultivation times were considerably shortened.