The heat balance of a U.S. Weather Bureau class A evaporation pan located near Tucson, Arizona, was calculated for selected periods during the interval July 26–31, 1963. The method used required two pans, one of which was insulated on the side and bottom to prohibit heat transfer through these surfaces. For 10 of the 14 observation periods, heat transferred into the uninsulated pan via the side or bottom accounted for 29% of the total heat added to the pan. In the remaining periods, 6% of the heat leaving the pan was transferred through the side or bottom. Elimination of this heat transfer in the insulated pan resulted in an average daytime water temperature 2°C lower than that in the standard pan and a reduction of the daily evaporation rate by 28%. The data show the necessity of taking into account the heat transfer through the walls of the pan, especially when evaporation of natural water bodies is estimated from pan evaporation data. The heat balance method described offers one way in which the magnitude of this heat transfer can be determined. (Key words: Evaporation; lakes; meteorology; hydrology.)