Discovery of seafloor volcanism west of Buldir Volcano, the westernmost emergent volcano in the Aleutian arc, demonstrates that surface expression of active Aleutian volcanism falls below sea level just west of 175Á9 E longitude, but is otherwise continuous from mainland Alaska to Kamchatka. Lavas dredged from newly discovered seafloor volcanoes up to 300 km west of Buldir have end-member geochemical characteristics that provide new insights into the role of subducted basalt as a source component in Aleutian magmas. Western Aleutian seafloor lavas define a highly calc-alkaline series with 50–70% SiO 2. Most samples have Mg-numbers [Mg# ¼ Mg/(Mg þ Fe)] greater than 0Á60, with higher MgO and lower FeO* compared with average Aleutian volcanic rocks at all silica contents. Common basalts and basaltic andesites in the series are primitive, with average Mg# values of 0Á67 (60Á02, n ¼ 99, 1SD), and have Sr concentrations (423 6 29 ppm, n ¼ 99) and La/Yb ratios (4Á5 6 0Á4, n ¼ 29) that are typical of island arc basaltic lavas. A smaller group of bas-altic samples is more evolved and geochemically more enriched, with higher and more variable Sr and La/Yb (average Mg# ¼ 0Á61 6 0Á1, n ¼ 31; Sr ¼ 882 6 333 ppm, n ¼ 31; La/Yb ¼ 9Á1 6 0Á9, n ¼ 16). None of the geochemically enriched basalts or basaltic andesites has low Y (<15 ppm) or Yb (<1Á5 ppm), so none show the influence of residual or cumulate garnet. In contrast, most western seafloor andesites, dacites and rhyodacites have higher Sr (>1000 ppm) and are adakitic, with strongly fractionated trace element patterns (Sr/Y ¼ 50–350, La/Yb ¼ 8–35, Dy/Yb ¼ 2Á0–3Á5) with low relative abundances of Nb and Ta (La/Ta > 100), consistent with an enhanced role for residual or cumulate garnet þ rutile. All western seafloor lavas have uniformly radiogenic Hf and Nd isotopes, with e Nd ¼ 9Á1 6 0Á3 (n ¼ 31) and e Hf ¼ 14Á5 6 0Á6 (n ¼ 27). Lead isotopes are variable and decrease with increasing SiO 2 from basalts with 206 Pb/ 204