A 2.2-m-thick lake bed composed entirely of diatomite was found in the southwestern arm of the Nafud sand sea, about 67 km east of Tayma in northwestern Saudi Arabia. Its stratigraphic position above eolian sand and below a large barchanoid dune indicates deposition during an exceptional period of pluvial climate in an otherwise arid sand desert. Radiocarbon-dated carbonate lake beds found in similar stratigraphic settings in other parts of the Nafud suggest that the diatomite was deposited in the late Pleistocene epoch sometime between 32,000 and 20,000 years ago. Diatomite of the same pluvial episode has also been found in the Afar region of Ethiopia. Four samples of the diatomite were analyzed for major chemical components and 32 minor and trace elements. Two of the samples appeared to be representative of the lake bed. These samples contained about 85 percent SiO2, 2.34 percent Fe2O3, 1 4 percent Al2O3, and 8.27 percent ignition loss, with only traces of CaO, P2O5, MgO, Na2O, and TiO2. The high content of silica and low contents of alumina, iron oxide, lime, and other constituents indicate low contamination. Analyses of samples from the two atypical units indicate a small amount of contamination, probably clay or mica, in the basal unit and addition of iron oxide and carbonate by ground water in an isolated green-colored unit. Chemically the Bi'r Hayzan diatomite compares favorably with commercial deposits from other parts of the world. Low bulk densities of 0.64 g/cm3 (30-40 lbs/ft3) and an average porosity of 63 percent for five samples also suggest the deposit is of industrial grade. Further studies are recommended to determine: (1) the size and extent of the deposit and (2) the suitability of the diatomite for commercial applications. As means to these ends, electrical-resistivity surveys following winter rains are suggested to determine both the extent and thickness of the lake bed beneath the overlying sand dunes. A complete species identification of the diatom association and further laboratory testing of physical properties of the diatomite also should be made. Further exploration in the Nafud is recommended to locate other possible diatomite deposits.