Abstract— <p>The work is devoted to the experimental study of <i>Arthrospira platensis</i> culture growth in a two-stage luminostat. A new method of flow culture is presented, which allows increasing the efficiency of light energy assimilation, as well as controlling the content of photosynthetic pigments in algal biomass. At the first stage, the growth of <i>A. platensis</i> culture at step 1 of luminostat in the batch mode was investigated. Due to high surface and spatial irradiance, there is a prolonged exponential phase on the growth curve. The maximum specific growth rate of the culture was 0.96 1/day and the chlorophyll <i>a</i> and C-phycocyanin content was low: 1.37 and 2.3%, respectively. The light energy passing through the first stage varied from 2.23 to 33.24% depending on the maintained density of the quasi-continuous culture. The reduction in light flux affected the maximum productivity of <i>A.&#xa0;platensis</i> at step 2 of luminostat, which was 0.08 g/(L&#xa0;day). In the second experiment, cultivation was carried out in a two-stage luminostat regime, when the nutrient medium sequentially flowed through both steps. It was shown that at the second stage, due to a decrease in irradiance, there was photoadaptation of <i>A. platensis</i> cells consisting in a slight increase in the proportion of chlorophyll <i>a</i> and a significant increase in C-phycocyanin: 13.6%. The pigment content was directly influenced by the uniformity of cell irradiation, being only 8% for the second stage, which led to a decrease in productivity and an increase in the content of C-phycocyanin. The findings allow us to recommend the two-stage luminostat for solving the problems of optimization in order to obtain microalgal biomass with desired pigment composition.</p>