The Eocene Casto pluton (Idaho) is a newly identified low-18O igneous rock, the first intrusive body related to caldera eruptions found to have this character. This pluton crystallized from a magma with a 6.7‰o range of oxygen isotope ratios. These values vary gradationally across the pluton. The Casto pluton is a high-silica granite within the extensional trans-Challis fault zone. It intruded into the basal parts of the Thunder Mountain and Van Horn Peak cauldron complexes after they collapsed, and its rocks are intrusive equivalents of the eruptive rocks of the cauldrons. Unaltered Casto samples have δ18O values that range from 2.7‰o to 9.4‰o. Mineral-mineral fractionations in these rocks exhibit typical magmatic values: quartz-orthoclase averages 2.2‰o and quartz-biotite averages 5.1‰o. Thus, parts of the pluton crystallized from a low-18O magma. The low-18O values are thought to have resulted by assimilation of up to 55% hydrothermally altered wall rocks. The low values are concentrated in the central part of the pluton, along the southern extension of the Thunder Mountain cauldron complex. Similar to other Tertiary low-18O igneous rocks in the American Cordillera, the Casto pluton crystallized from a high-silica magma emplaced within an extensional tectonic environment, and it is intimately associated with ash-flow eruptions and complex caldera collapse structures.