St. Paul Island is the youngest volcanic center in the Bearing Sea basalt province. We have undertaken a field, petrographic, and geochemical study of select St. Paul volcanic rocks in order to better understand their differentiation; specifically, to test the hypothesis that magmas erupted from individual Bering Sea basaltic volcanoes are not related by shallow-level processes such as crystal fractionation. Petrographically, all of the St. Paul volcanic rocks are olivine-, plagioclase-, and clinopyroxene-phyric. Textural features and modal contents of olivine phenocrysts, however, vary widely throughout the spectrum of basalt compositions. Although differing in size and abundance, olivine phenocrysts in all rock compositions are euhedral and commonly skeletal, suggesting rapid growth during ascent or eruptive quenching. None, however, display reaction textures with surrounding groundmass liquid. Compositionally, the St. Paul volcanic rocks are basalts and tephritic basalts and all have high contents of normative nepheline (8% to 16%). Concentrations of many major and incompatible trace elements display no clear correlations with bulk-rock SiO 2 and MgO contents or modal abundances of phenocrysts, suggesting that much of the compositional diversity of these magmas reflects variable mantle sources and degrees of partial melting. Similarly, chondrite-normalized REE patterns show variable degrees of light REE enrichment (La n=70–90) that do not correlate with bulk-rock mg-numbers. In contrast, concentrations of compatible trace elements (Ni, Cr, and Co) are positively correlated with MgO contents and modal percentages of olivine phenocrysts. Maximum forsterite contents of olivine phenocryst cores in most St. Paul rocks decrease with decreasing bulk-rock mg-number and are similar to the calculated equilibrium range. This is evidence that the high mg-numbers are magmatic and do not result from olivine accumulation. Instead, major and compatible trace element mass balance calculations support derivation of the low mg-number lavas from the high mg-number lavas mainly by olivine fractionation, which, in turn, implies that St. Paul magmas may have temporarily resided in crustal magma chambers prior to eruption.