The red microalga Rhodella reticulata, a potential source of bioactive substances, was the subject of study of the irradiance and temperature effects on growth rate and biochemical composition of algal biomass. The optimum temperature for growth decreased from 28 to 26°C with increasing light intensity from 260 to 520 µE/(m2 s). The maximal growth rate was 0.21/day at 28°C and lower light intensity (260 µE/(m2 s)). Variations in these parameters also affected the fatty acid productivity, and proteins and carbohydrates content. At 34°C and high light intensity the quantity of carbohydrates was 1.16-fold higher than the quantity at the optimal temperature and low light intensity. Protein content was the highest at lower temperatures for both light intensities. Fatty acid profile showed the highest percent for the polyunsaturated eicosapentaenoic acid (EPA) at 28°C and both light intensities (46% from the whole fatty acid content), an important feature for this strain. This is a prerequisite for use of EPA as a supplement in food industry.