Circum-Pacific intra-oceanic island-arc lavas have a mean 87Sr/86Sr of .70355. Associated marginal basins have a mean 87Sr/86Sr of .70311. Significant hemispheric variations are noted for both island arcs and back-arc basins. In the north Pacific, mean 87Sr/86Sr for island arcs (.70335) and back-arc basins (.70287) is less radiogenic than counterparts in the south Pacific (.70374; .70336). This hemispheric variation is not related to the age of sea floor being subducted; therefore, partial fusion of sediments and/or altered crust are not likely to be responsible. Back-arc–basin magmas are generally depleted in radiogenic strontium relative to those of the associated arc, but arc-basin systems are enriched or depleted systematically. Arcs around the western Pacific show regional variations similar to that observed for alkali basalts of the Pacific basin. Both show enrichments in radiogenic strontium near Samoa and become progressively depleted away from it. Least-radiogenic samples from continental-margin volcanics indicate their sources are more radiogenic in the southern circum-Pacific. These regional variations in 87Sr/86Sr in island-arc and marginal-basin systems suggest that: (1) Regional isotopic variations noted for the mantle beneath the Pacific basin are maintained across convergent plate boundaries. (2) The source regions of marginal-basin basalts are isotopically similar to that of mid-ocean ridge basalts, while those of island arcs are similar to those of intra-plate ocean-island basalts. (3) The sources of island arcs and marginal-basin magmas lie in the mantle.