Twenty-nine samples of bauxites from different locations were analyzed for Th and U by gamma-ray spectrometric, alpha counting, and wet chemical methods. The Th concentrations range from 5.0 to 131 p.p.m. and average 48.9 p.p.m. The U concentrations range from 2.7 to 26.7 p.p.m. and average 11.4 p.p.m. The Th-to-U ratios range from 1.5 to 20.9, with an average value of 5.1. Zr determinations on some of the bauxites gave values ranging from 0.02 to 0.65% Zr, and averaging 0.09% Zr. The Th, U, and Zr contents of the bauxites are related to the type of source rock. Bauxites derived from nepheline syenites contain more Th and U than bauxites derived from other rock types; bauxites derived from basic igneous rocks have the lowest Th and U concentrations, and bauxites derived from carbonate rocks have the lowest Th-to-U ratios. The greatest concentrations of Zr were found in bauxites derived from nepheline syenites, and bauxites derived from carbonates contained the lowest concentrations of Zr. A study of samples from alumina plants treating Surinam and Arkansas bauxites shows that during the process, over 70% of the Th and U in these 2 bauxites is concentrated into the red mud, and the alumina contains very small amounts of these elements. This study also indicated that much of the Th and U in bauxites occurs in either primary or secondary resistate minerals.