In any study of lake metabolism, one of the basic considerations is primary productivity. How to measure this parameter with any degree of confidence in such large water masses as the Great Lakes is a challenge. It has been proposed That a reliable estimate could be obtained if there were a definite relationship between the chlorophyll a content of the photic zone and the amount of solar radiation. A series of experiments were conducted from July 20 to August 20, 1959, at a single station located at the lower end of the West Arm of Grand Traverse Bay, Michigan. Water samples from surface to 25 meters were taken at 5-meter intervals. Radioactive carbon (C14) was introduced into each sample bottle. Bottles were resuspended at the sampling depth for 24 hours. A similar series was incubated for 4 hours under an illumination of 500 foot candles and surface water temperatures. An analysis for chlorophyll a was made for each sampled depth. During the month of investigation, the chlorophyll a content varied from 0.7 to 4.2 mg/m3 with a mean of 1.9 mg/m3. In general there was a concentration of pigment at or below 20 meters. When the chlorophyll a calculated productivity values were compared with the in situ values, there was with one exception a consistent underestimation. This amounted to approximately 20%. The consistency indicated some bias either within the experiments or the method proposed. From the data it was estimated that primary productivity in Grand Traverse Bay amounted to 67 gC/m2/year. Further investigation was recommended.