The Bishop Tuff is a large rhyolitic ash flow which erupted from Long Valley caldera in eastern California and contains abundant sanidine cryptoperthite phenocrysts, which can be used to constrain the time between the two major cooling units. The cryptoperthites (bulk composition Or66Ab33An1) have exsolved in two principal stages, followed by a third, minor stage. The earliest exsolution, which produced patches of relatively wide lamellae (100--800 nm) without wedge--shaped ends (WSE), occurred by coherent nucleation and was probably related to a high flux of water associated with fumarolic activity in this area. The spacing of these lamellae shows no correlation with depth or thermal history. The second major stage of exsolution occurred by spinodal decomposition, which produced lamellae with WSE having spacings of from 80 to 170 nm, and the spacing of these lamellae varies with depth in a pattern consistent with the two known cooling units. Heat flow calculations indicate that this pattern of lamellar spacings is consistent with an inverval of about 1.0--2.0 years between the eruption of these two cooling units. ¿ American Geophysical Union 1988