Abstract: A light chamber was constructed for the measurement of CO2 assimilation by plant communities. The chamber was illuminated either by sunlight or by a group of incandescent lamps. The temperature of the chamber was kept constant by flowing water and adequate inside ventilation was ensured by an electric fan. To simulate field conditions, similar plants were placed outside the chamber, or, in the case of artificial illumination, the side walls of the chamber were covered with mirrors. The CO2 assimilation rate was measured by the compensation method, in which the air sample was being drawn continuously through the measuring apparatus, the change of CO2 content due to photosynthesis in the chamber was detected colorimetrically and CO2 was added from time to time to keep its concentration at 0.03%, i.e., the CO2 concentration of the atmosphere. From the amount of CO2 added in a given time, the rate of assimilation was calculated. The CO2 assimilation rate of plant communities with different leaf area indices were measured. The results obtained with wheat, rice, perilla and sunflower were shown in Figs. 1–4. The CO2 assimilation-leaf area curves are similar in general form, direct proportionality exists only at lower leaf areas, and when the leaf area increases up to a certain value, the CO2 assimilation rate gradually ceases to increase with it. The optimal leaf area, i.e., the leaf area at which the assimilation rate reaches its maximum, is about 4 with rice and wheat, 5–6 with Perilla and 2–3 with sunflower. The figures obtained with rice and wheat were in accord with those obtained by calculation based on light intensity distribution and the rate of assimilation of individual leaves as a function of light intensity. The causes of the difference between different plant species were discussed in relation to the arrangement of leaves. The maximum assimilation rates of communities of wheat and rice were about 3.5 and 4.2 g CO2/M2 land surface/hr, respectively, which are more than twice as much as those of a single layer of leaves of the same area. This shows the benefit of plant com- munities with many layers of leaves in ensuring better utilization of strong light.