This batholith was intruded 162 + or - 2 m.y. ago into metamorphic Palaeozoic rocks and Triassic rocks of the Klamath Mountains, N California. It is gradationally zoned from two- pyroxene gabbro in the deepest exposed portion to hornblende-biotite granite at shallowest levels, corresponding with two distinct chemical trends in the mafic minerals. Pyroxene-bearing rocks, as well as being structurally lower, are richer in Mg, Ca, Cr, Ni, Co and Sc than the higher pyroxene-free rocks which are relatively richer in Al, Na, K, Sr, Zr and Rb. These two chemical trends can be explained by either two coexisting magmas in a single chamber or crystallization under a vertical aH2O gradient, the latter hypothesis being preferred. Major-element modelling suggests that the trends can be explained by crystal fractionation of a single parent which became successively richer in H2O in the highest levels of the pluton; this upward H2O-enrichment led to plagioclase stability at lower T and consequent enrichment of the magma in Al and alkalis. Trace-element abundances show that minor accumulation of clinopyroxene, olivine and chromian spinel affected the lower part of the pluton and that minor influxes of basaltic magma affected the upper, H2O-rich part of the pluton. These basaltic magmas were trapped in the interface between the lower, dense, H2O-poor magma and the upper, H2O-enriched magma. As these basaltic liquids cooled and crystallized, they mingled with the H2O-rich magma of the pluton.-R.A.H.