Although the volumetrically dominant lava in the Aleutian arc is basaltic, its petrographic, geochemical, and isotopic character is variable, and the petrologic significances of the different basalts have been much debated. To evaluate possible petrogenetic relations, all available petrographic, major-, trace- and rare-earth-element and isotopic data from Aleutian basalts (n = 205) have been collected. On the basis of MgO content, basaltic (<52 wt % SiO2) lavas have been divided into three major classes: high-magnesia basalts, transitional basalts, and low-MgO basalts. Low-MgO basalts have less than 6% MgO and constitute 67% of the analyzed basalts. Using Al2O3 content, this basalt class has been further divided into two subclasses. Low-alumina basalts, the less abundant class (15% of the low-MgO basalts), have less than 18 wt % Al2O3 and 450 ppm Sr and high TiO2 (>1.0 wt %), FeOt (>9%) and incompatible element abundances. High-alumina basalts (≥18 wt % Al2O3) are characterized by low TiO2 (≤1 wt %) and ≤9% FeOt, low incompatible- and compatible- element contents and high Sr abundances (≥450 ppm). Transitional basalts are defined as basalts with between 6 and 9 wt % MgO. High-magnesia basalts (MgO >9 wt %) constitute 10% of the analyzed Aleutian basaltic lavas and occur at only a few volcanic centers. Fractionation of high-magnesia basaltic magma is unlikely to produce magma compositions similar to high-alumina basalts. The compositional data are, however, consistent with assimilation of lithospheric material by high-alumina basaltic liquid to produce high-magnesia basaltic magmas and generation of low-alumina basalts by crystal fractionation of these hybrid liquids. Within this context, high-alumina basaltic magmas are considered primary.