Chemical data from the Inyo volcanic chain of eastern California illustrate that the most recent eruption (about 600 years B.P.) sampled at least two separate magmatic systems. Two rhyolitic lava types were produced in the eruption, one coarsely porphyritic (CP) and one finely porphyritic (FP). These two lavas are chemically, mineralogically, and texturally distinct from one another, are physically intermixed, and are distributed unequally among three lava flows. The FP lava comprises a range of compositions that form coherent but discontinuous trends on variation diagrams. Quantitative major and trace element modeling indicates that the trends were not produced by fractionation of the observed phenocryst assemblage, but rather that the high-silica end-member could have been produced from the low-silica end-member by factionation of an equivalent hydrous assemblage. This and compositional discontinuity in the trends could indicate that two slightly zoned FP end-members were generated at higher water pressure, and then back-mixed to form the present array of FP lava compositions. In contrast, chemical distinctions, physical nature, and chemical modeling suggest that the CP and FP magmas evolved separately along distinct, and possibly parallel, liquid lines of descent. Since these are the most recent lavas to erupt in the Long Valley Caldera, this suggests that at least two chemically isolated magmatic systems are present beneath the region. The three lava flows produced in the 600-year-B. P. eruption lie along a north-trending line that crosses the northwestern boundary of the caldera. The CP lava is not found outside the caldera, increases in abundance toward the caldera center, and shows marked physical and chemical similarity to 100,000-year-old lava erupted in the caldera's northern topographic moat. The more crystalline nature of the CP lava, coupled with its similarity to older Long Valley lava, may indicate that it came from a source within the caldera, possibly the same magma body as the one detected seismically beneath the northwest moat of the caldera. ¿American Geophysical Union 1987