Abstract The 1954 uplift of Urvina Bay on Islá Isabela exposed porphyritic lava that erupted from a moderately shallow magma chamber onto the southwestern flanks of Volcán Darwin at least 1200 years ago. The lava contains: (1) 20–27% plagioclase xenocrysts, glomerocrysts, and phenocrysts; (2) 0.2–2.7% clinopyroxene xenocrysts and phenocrysts; and (3) 0.2–0.8% olivine phenocrysts, glomerocrysts, and xenocrysts. Whole-rock major-element data corrected for phenocrysts, glomerocrysts, and xenocrysts indicates that the lavas are moderately evolved, (Mg# = 50–52 mol%) transitional basalt. Wholerock chondrite-normalized La/Lu (3.9–5.0) values are similar to those for ocean-island tholeiites and alkali basalts on the western side of the Galápagos Islands platform. Disequilibrium textures for xenocrysts and variations in mineral chemistry suggest that xenocrysts and some glomerocrysts crystallized in equilibrium with melt that was less evolved than the groundmass. We propose that primitive magma, containing olivine (Fo > 80), endiopside, and calcic plagioclase, was injected into a subvolcanic chamber and mixed with more evolved magma which was crystallizing olivine (Fo < 80), An-poor plagioclase, and clinopyroxene phenocrysts. Resorption of xenocrysts probably accompanied melting during magma ascent. Homogeneous compositions of xenocrysts and the scarcity of reverse zoning in phenocrysts imply that eruption may have been triggered by injection of hotter, more basic magma.