Thirty soil profiles located in the Idaho batholith were sampled, and the clay fractions were x-rayed for mineral identification. Kaolinite and halloysite predominate in all residual soils having adequate drainage and are considered to be the clay minerals formed in equilibrium with the present soil environment. Mica, probably originating from the weathering of biotite and (or) feldspars, is found abundantly in the clay fraction. Montmorillonite distribution is restricted either to relatively dry sites at lower elevations or to sites having poor drainage. Amorphous free SiO2 and Al2O3 contents are greater in the kaolinitic soils than in the montmorillonitic soils, perhaps due to the more moist soil environment. Free SiO2:Al2O3 molar rados exceeding 10 favor formation of montmorillonite. Soils containing mainly kaolinite have SiO2:Al2O3 ratios averaging half the SiO2:Al2O3 ratios in montmorillonite clays. The clay fraction of soils formed from loess, common in the northern batholith, show indistinct x-ray peaks and are assumed to be largely noncrystalline.