In continuous cultures of Spirulina platensis under photoautotrophic conditions, the operation mode to maintain constant cell concentration is examined using culture apparatuses with light path lengths of 0.02 and 0.156 m. The values of dilution rates in the continuous cultures are determined by employing specific growth rates of the cells corresponding to light intensity distribution in liquid medium. When incident light intensity is fixed at 25, 50 or 400 W m(-2), it is found that the cell concentration in the continuous culture is kept almost constant, and agrees fairly well with the prescribed value of the cell concentration in the range of 0.09 to 2.43 kg m(-3). It is also demonstrated that the operation mode is valid to obtain stable cell concentrations in the continuous cultures associated with the change in incident light intensity ranging from 25 to 400 W m(-2) during the cultures.