Optimally attenuated larvae of Schistosoma mansoni (20 krad) are incapable of migrating farther than the lungs, and they induce high levels of resistance in mice to a subsequent challenge infection. The effect of gamma radiation on the parasite is described and related to the migratory pattern of attenuated larvae. Scanning electron microscopy revealed marked morphological differences between normal and irradiated larvae at the lung stage of development. The latter exhibited random constrictions, probably resulting from contraction of circular muscle fibers, at intervals along the length of the body and showed subtle differences in motility. We suggest these abnormalities account for persistence of attenuated larvae in the skin-draining lymph nodes and lungs, 2 events that are instrumental to the induction of protective immunity in this vaccine model.