The Bishop Tuff represents a single eruption of chemically zoned rhyolitic magma. Six whole rock samples spanning the compositional and temperature range yield initial 87Sr 86Sr of 0.7060-0.7092 and ??18O of 5.9-10.3???. Six constituent sanidines yield smaller ranges of initial 87Sr 86Sr of 0.7061-0.7069 and ??18O of 6.7-7.9. In contrast 143Nd 144Nd ratios for the six whole rocks and two constituent magnetites exhibit negligible variation with a mean of 0.51258 ?? 1. These data are used to show that the phenocrysts were precipitated from an already chemically zoned liquid, that the zoning process involved negligible assimilation of, or exchange with, country rocks and that the extreme Sr and O isotopic disequilibria are probably the result of post-eruptive interaction with meteoric water. The parent magma had ??Nd = -0.9, ??Sr = +23 and ??18O = 7??? and was formed from mantle-derived magmas and/or melts of lower crustal rocks isotopically similar to parts of the Sierra Nevada Batholith. ?? 1984.