Electron-probe microanalysis for Ca, K, and Na in detrital feldspar can be used to determine the ultimate origin of plagioclase in arkosic sandstone. In volcanic rocks, K content of plagioclase increases with decreasing Ca content. The plagioclase of metamorphic rocks contains a negligible amount of K, and metamorphic plagioclase can be distinguished readily from volcanic plagioclase. Plutonic plagioclase contains an intermediate amount of K, and its compositional range overlaps in part the compositional range of both volcanic and metamorphic plagioclase. Hundreds of detrital plagioclase grains from various arkosic sandstones were analyzed for Ca, K, and Na. These detrital grains may be placed into seven provenance groups: volcanic; plutonic; metamorphic; volcanic or plutonic; volcanic or granophyre; plutonic or metamorphic; and plutonic, low-rank metamorphic or authigenic. The electron microprobe is a valuable tool for provenance study of detrital plagioclase. Microanalysis of detrital plagioclase can be used to detect provenance variations in sedimentary basins of active continental margins and might have bearing on plate-tectonics models. The electron microprobe deserves increased use by sandstone petrologists.