Strong grating formation in pure silica-core fibers by use of 193-nm ArF-laser radiation is reported. Unsaturated refractive-index changes of Dn∼0.3x10(-3) were observed in nontreated fiber, and changes of Dn∼0.5x10(-3) were observed in fibers with a high hydroxyl concentration. Possible mechanisms of photosensitivity in pure silica-core fibers are discussed.