Photosynthesis in Cylindrotheca closterium was greatest in 1-day-old cells (2-4 � O2 per 106 cells per hour), declining to about 50% of this at 3 days and to about 10 % at 14 days. Due mainly to a decrease in the intensity of photosynthesis the ratio of photosynthesis to respiration fell from 6-8 at 1 day to 34 at 3 days, and to 0.6-1.6 at 14 days. The decline in photosynthesis started before phosphate or nitrate in the culture medium was exhausted, but was accompanied by a pH change from 7.4 before inoculation to 8.4 at 1 day and 9.2 at 3 days and thereafter. White light of 1000-1500f.c. (6.4-9.6 mW/cm2) saturated photosynthesis, the compensation point varying from 50 to 100 f.c. (0.3-0.6 mW/cm2). In blue light similar in spectral composition to that found at 10m below the surface of the ocean, the compensation point was 0.5-0.6 mW/cm2.