Isla Tortuga is a recently formed volcanic island located on a fracture zone of an active spreading trough in the Gulf of California. The volcano is composed of tholeiitic basalt lava flows and vitric tuffs and minor volumes of tholeiitic andesite. It was built in at least two stages by a northward-migrating point source of volcanism. The latest stage of activity culminated in caldera collapse, extrusion of the surficial flows, and the formation of a lava lake. The suite of basalts and Fe-Ti–rich basalts on Tortuga is chemically very similar to suites of ocean-ridge tholeiite. The chemical variation observed in the Tortuga basalts can be explained by removal of the low-pressure phase assemblage Pl + Cpx + Ol from a relatively “primitive” (high Mg/Fe) tholeiitic basalt parental magma. The chemical variation of coarse-grained basaltic pegmatoidal segregations, which occur in thick flows on Tortuga, is similar to that of the basaltic lavas. These pegmatoids have fractionated in situ and have produced SiO2-rich liquids (interstitial glass) by the separation of Pl + Cpx + Ol + ilmenomagnetite and/or ilmenite from basaltic liquids. The most primitive Tortuga basalts are similar to ocean-ridge tholeiite in 87Sr/86Sr ratio and large-ion lithophile element abundance, but have higher sodium contents. This may be due to halite assimilation by the Tortuga basalt liquids.