Two types of digital control algorithm were experimentally examined for the control of yeast cell-mass concentration in a CSTBR(continuous stirred tank bioreactor). One of them is a high gain proportional control resulted from highly output weighted quadratic cost regulator. The other is a stochastic adaptive control named ELS-STR, which is a minimum variance controller with an extended least-squares method as parameter identifier. On-line esti-mation of the state variable, cell mass concentration, was carried out by measuring optical censity of the fermentor broth. From experimental results, it was known that the performances of ELS-STR were better than that of conven-tional PID controller in cell mass control case.