The measured magnetic susceptibilities of rocks from more than 600 outcrops in the southern Idaho batholith have a good spatial correspondence with the observed intensities of magnetic anomalies. Koenigsberger ratios are low (≤0.3) in 75% of cored samples, indicating that remanent magnetizations are unimportant relative to induced magnetizations (which are related to susceptibilities) in aeromagnetic anomaly interpretations in this region. Extremely low susceptibilities (&khgr;25¿10-5) within the eastern and western parts of the batholith are the sources of positive aeromagnetic anomalies; the associated lithologies include Mesozoic tonalite and granodiorite, as well as a diverse Tertiary group of epizonal stocks and batholiths. Intense meteoric-hydrothermal activity associated with the emplacement of these Tertiary plutons caused a reduction in &khgr; in some rocks, although coarse igneous magnetite is surprisingly resistant to hydrothermal corrosion and oxidation. However, under conditions of lower water/rock ratios, a significant increase in &khgr; commonly occurred in the leucocratic plutons, generally reflecting growth of hydrothermal magnetite. Because the measured susceptibilities do not necessarily represent more than the uppermost part of the 14- to 20-km thick crust that gives rise to magnetic anomalies, an apparent disordance between susceptibility measurements and susceptibilities inferred from aeromagnetic data can occur. Such disordance patterns occur near the margins of the batholith, where granitic rocks are thin, and also in hydrothermally altered regions, where subsurface Tertiary plutons are present. Magnetic models suggest that the Idaho batholith thickens to ~20 km in its east-central part, which is underlain by enormous Tertiary intrusive bodies.