We studied the effect of serial transfers on solventogenesis capacity and spore-forming ability by Clostridium acetobutylicum ATCC 824 in two culture media. In reinforced clostridial medium (RCM), an abrupt decrease of the ability to ferment glucose and produce solvent occurred after the 10th to 12th transfer and spore formation failed after the 25th transfer. When 1.5 g/l of acetic and butyric acids were added to the RCM, however, the capacity to produce a normal level of solvents was not lost, whereas spore-forming ability was lost after the 23rd transfer. The shift from solventogenesis to acidogenesis did not occur in chemically defined medium (CDM) even by the 40th transfer. The dramatic decrease of solventogenesis in the degenerate culture was coupled with acetate/butyrate CoA transferase and acetoacetate decarboxylase deficient activities.