Abstract: Based on the continuous fermentation process using a group of mutants of clostridium acetobutylicum,a mathematical model for such kind of mixed cultures was proposed.It can be used to determine the staying period of each of substantially used mutants with a given superior concentration in a continuous fermentation system.Then the yield of each mutant can be calculated using daily analysis data.By the comparison of yields of various mutants,better mutants,which have higher yields and more stable fermenting activities,can be selected.