Measurement of the magnetic susceptibility of more than 6000 samples of granitic rock from the Mariposa 1° by 2° quadrangle, which crosses the central part of the Sierra Nevada batholith between 37° and 38°N latitude, shows that magnetic susceptibility values are above 10⁻² SI units in the east and central parts of the batholith and drop abruptly to less than 10⁻³ SI units in the western foothills. In a narrow transitional zone, intermediate values (10⁻³ to 10⁻²) prevail. Magnetic susceptibility appears to decrease slightly westward within the zones of both high and low values. Magnetic susceptibility in plutonic rocks is chiefly a function of the abundance of magnetite, which depends, in turn, on the total iron content of the rocks and their oxidation ratio. Lower magnetic susceptibilities of felsic members of Sierran intrusive suites and of some felsic rock units relative to adjacent mafic rock units commonly reflect differences in total iron content, but the differences of magnetic susceptibility that define the regional pattern generally are much larger and are determined chiefly by the oxidation ratios of the rocks. The relatively unaltered condition of the samples and restriction of δ¹⁸O to the range of +7 to +10.3‰ indicate that neither hydrothermal fluids nor subsolidus alteration were important in modifying oxidation ratios. Correlations of magnetic susceptibility with initial ⁸⁷Sr/⁸⁶Sr suggest that oxidation ratios have been inherited from the source regions for the magmas from which the rocks crystallized. Reduction of Fe³⁺ to Fe²⁺ by organic carbon or other reducing substances may also have affected magnetic susceptibility.