Primary productivity of phytoplankton, measured as rate of carbon-14 uptake, was increased by adding iron and other nutrients to lake water, but was not increased by adding nutrients in the absence of iron. Concentrations of chelated iron ranging from 0.010 to 5.0 parts of iron per million increased primary productivity. Iron as a nutrient limiting primary productivity is related to the morphometry, physicochemical characteristics, and low productivity of the lake.