Primary productivity of reef—building algae was studied by putting samples from the reef in a closed system and measuring oxygen exchange in the light and in the dark. Gross productivity determined for 32 samples in full sunlight had a mean value of 0.048 mg O2cm—2hr—1. Photosynthesis was found to increase with the logarithm of light intensity up to 1,000 ft—c and was constant between 1,000 and 8,000 ft—c. Rates of gas exchange in flowing water showed no correlation with water velocity but were greater than rates in still water. Daily patterns of photosynthesis were calculated for populations of calcareous algae living on the submarine faces of the windward sides of atolls. During most of the daylight hours light is probably not a limiting factor for photosynthesis in these populations. Calculated productivity of various calcareous algal zones indicates that these do not contribute significantly to overall reef production on atolls of the northern Marshall Islands. Island reefs are less productive than previously studied inter—island reefs.