The oxygen and carbon-14 methods were used to measure photosynthesis in Clear Lake, Iowa during 1958 and 1959. Differences in the rates of photosynthesis at widely separated stations were generally small. Daily variations in the rate of photosynthesis were not greater than two-fold. The correlation between the rate of photosynthesis and the incident illumination was 0.81, and the efficiency of utilization of incident light energy was 0.72 per cent. The net gain of organic matter at the phytoplankton level during the period May 1 to November 1 was equivalent to 3480 pounds of glucose per acre
