Data concerning photosynthesis in the Great Lakes are very scanty. There have been only two research programs which have dealt directly with photosynthesis in the Great Lakes. One of these programs attempted to gain some insight as to the distribution of photosynthesis in western Lake Erie. The other program attempted to develop and evaluate a shipboard method for estimating photosynthesis. Some additional inference concerning photosynthesis can be made using known concentrations of chlorophyll in Lakes Superior, Michigan, Erie, and Ontario. No data are available for Lake Huron. The assumptions made using chlorophyll to calculate photosynthesis are very broad and therefore interpretation is somewhat tenuous. When results for calculated photosynthesis using all methods are compared, two points are apparent: 1) the range of photosynthetic activity in all lakes is very large, and 2) western Lake Erie is very different from the other Great Lakes. It is more productive than many smaller lakes which are considered to be highly productive. The Great Lakes are important as a natural resource and as basins in which controlled experiments can be performed. Perhaps the very paucity of data indicates the exciting future which lies ahead for those individuals who study photosynthesis and other biotic events in these large lakes.