Data collected as part of the International Biological Program from 43 lakes and 12 reservoirs, distributed from the tropics to the arctic, were subjected to statistical analysis to establish which factors are important in controlling production and how they are related. In the whole body of data, variables related to solar energy input have a greater influence on production than variables related to nutrient concentration; in lakes within a narrow range of latitude, nutrient‐related variables assume greater importance. Morphological factors have little influence on productivity per unit area in either case. Chlorophyll a concentration is a good indicator of nutrient conditions and when combined with an energy‐related variable constitutes a good estimator of primary production.