The effects of the phytohormone indole-3-acetic acid and various conditions of illumination on the growth of Parachlorella kessleri were investigated. Two variants of illumination: continuous and photoperiod 16/8 h (light/dark) and two concentrations of the phytohormone – 10^-4 M and 10^-5 M of indole-3-acetic acid were used in the experiment. The results of this study show that the addition of the higher concentration of indole-3-acetic acid stimulated the growth of P. kessleri more efficiently than the addition of the lower concentration of indole-3-acetic acid. This dependence can be observed in both variants of illumination. Increased biomass productivity was observed in the photoperiod conditions. Both the addition of the phytohormone and the conditions of the illumination had an impact on the number of P. kessleri cells. An increased number of cells was observed under the conditions of continuous illumination. This result has shown that the continuous illumination and the higher concentration of the phytohormone stimulated the growth of P. kessleri more effectively than the shorter duration of light (16/8 h (light/dark)).