Plagioclases in the 1974 high-Al basalt from Fuego Volcano have δO18 values of +6.0 to +8.5 per mil. Meteoric water cannot have played a significant role in Fuego’s magma. Large, weakly zone clear phenocrysts had δO18 values in the accepted mantle range, while patchyzoned and oscillatory-zoned plagioclases inferred to have formed later and shallower levels have slightly heavier oxygen isotopic ratios.