An empirical approach has been taken to develop a geothermometer based on plagioclase-magmatic liquid equilibrium. Compositions of coexisting plagioclase and liquid (glass) obtained by electron microprobe analysis of quenched samples from equilibrium melting experiments of natural granitic rocks at water pressures of 0.5 and 1.0 kilobars have been used along with data from the equilibrium experiments of Bowen (1913, 1915), Prince (1943) and Yoder et al. (1957) to calibrate this geothermometer. Applications of this geothermometer to natural occurrences demonstrate that it can provide useful information on temperature of equilibration of coexisting plagioclase and liquid in rocks ranging in composition from basalt to rhyolite. The plagioclase geothermometer is in good general agreement with other geothermometers wherever these are applicable. Where temperatures are known from other sources it can be used to predict the equilibrium compositions of plagioclase in magmas as well as to provide a rough estimate of water pressure.