From an engineering point of view, the effect of light intensity distribution on the stability of growth rate should be taken into account in designing effective photobioreactors and sustaining stable growth rates. In the experiments described here, in order to keep operational parameters at an almost constant level, a semi-continuous culture method was developed for cultivation of photosynthetic cells under defined light intensity distributions. In the semi-continuous culture, a portion of culture broth containing grown cells was repeatedly replaced with the fresh medium at a predetermined time interval to maintain the cell concentration and the volume of the broth constant at their initial values. Under illumination from one and both sides, photosynthetic cells were cultivated in a flat plate photobioreactor with various light path lengths. The results obtained showed that stability of the growth rate strongly depended on the distribution of light intensity and the ratio of light intensity in the illuminated to that in the dark zone inside a photobioreactor. These parameters should be taken into consideration for stable cultivation of photosynthetic cells.