A basanitoid dredged from near St. Paul's Rocks represents the first reported occurrence of an abyssal lava containing ultramafic xenoliths and highpressure xenocrysts. The xenoliths include three separate populations. A suite of clinopyroxene-poor spinel-harzburgite and spinel-lherzolite inclusions have tectonite textures and highly refractory mineral compositions; these nodules probably represent residua from partial melting events unrelated to the host basanitoid. One harzburgite inclusion apparently accumulated from a tholeiitic magma and is also an accidental inclusion. Ultrabasic mylonites comprise a third xenolith group whose members are comparable to lithologies exposed on St. Paul's Rocks. A diverse xenocryst assemblage including labradoritebytownite, andesine, high-Ca pyroxene, low-Ca pyroxene, aluminous Cr-spinel, Cr-free aluminous spinel and Fe-Ti oxides, suggests at least two petrogenetic affinities. Fe-rich low-Ca pyroxene and Carich plagioclase, together with the cumulate harzburgite xenolith, indicate the presence of a differentiated tholeiitic suite in the area. Most xenocrysts, including Ca-rich pyroxene, spinel and andesine have alkalic affinities consistent with crystallization at elevated pressure over a wide range in temperature. Both spinel populations are interpreted as cognate precipitates with Cr-free Al-spinels crystallizing above about 10 kb and aluminous Cr-spinels co-precipitating with plagioclase at lower pressures. Compared to the olivine-dominated, low-pressure phenocryst assemblage, extraction of the generally basaltic, aluminous clinopyroxene-dominated assemblage at high pressure should have a minor effect on the major element differentiation of the host basanitoid magma.