A direct application of the C14 method of estimating photosynthesis in small lakes to large bodies of water would require the establishment of a fixed buoy at each sampling station. A modified method requiring only one fixed station has been proposed by other workers. At the one fixed buoy the photosynthetic rate of the surface plankton is measured at a series of depths in the photic zone in order to determine the relationship of changing light with depth to photosynthesis per unit population for the particular lake in question. At this station and all other sampling stations the photosynthetic rates of plankton collected from different depths are measured under constant light conditions in order to arrive at a relative measure of the population densities. A shipboard daylight aquarium is used to estimate daily photosynthesis of surface phytoplankton. This information is combined to calculate photosynthesis at each depth and integral photosynthesis for each station. Field tests of this method were made in the summer of 1959 at single stations in Douglas Lake, Cheboygan County, Michigan, and in the west arm of Grand Traverse Bay near Traverse City, Michigan. Comparison of the calculated values for integral photosynthesis with parallel in situ curves showed no signficant difference in the Douglas Lake experiments, whereas in Grand Traverse Bay the modified method consistently underestimated the in situ values. An analysis of the data indicated that light adaptation by the Grand Traverse Bay plankton was responsible for the difference between methods and a further modification of the method was proposed to correct for this factor.