The tectonic position of Aleutian arc volcanic centers and their magmatic differentiation trends (calc-alkaline or tholeiitic) appear to correlate. From 160¿W to 174¿E, the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates. The tholeiitic volcanoes are large, primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow, closed system differentiation. The calc-alkaline volcanoes are smaller, more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult. Differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers. The chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma, probably derived from mantle peridotite. Conditions for the derivation of the two trends through crystal fractionation support the proposed model. The tholeiitic magmas show characteristics (i.e., no hydrous phenocrysts, Fe enrichment trend, parallel REE patterns, vitrophyric andesites and dacites) consistent with low-pressure, high-temperatures crystallization in large shallow magma chambers. The calc-alkaline magmas show characteristics (i.e., some hydrous phenocrysts, no Fe enrichment trend, nonparallel rare earth patterns, porphyritic lavas) consistent with higher pressure and lower temperature of crystallization than the tholeiitic series. Tertiary plutons also show both calc-alkaline and tholeiitic trends and appear to be chemically similar to the Quaternary volcanoes. The larger plutons are calc-alkaline and probably represent the extensive (100 km2) magma chambers of the small calc-alkaline volcanoes. The small, shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series.