Two young mentally defective children were infected with a small number of embryonated eggs of the dog ascarid, Toxocara canis. They remained asymptomatic clinically but developed eosinophilia which persisted for more than 13 months. Tissues from mice infected experimentally with the same dose of dog ascarid eggs were examined at intervals up to one year after infection. In the early stages of infection the larvae were found principally in the liver and lungs, but in the later stages they were most numerous in the brain. It seems not improbable that some of the neurologic manifestations associated with ascariasis may be due to actual presence of Toxocara larvae in the tissues of the central nervous system, rather than to toxic or allergic effects of Ascaris lumbricoides. This new evidence, added to previous observations on the effect of dog and cat ascarids in children, re-emphasizes the need for ridding household pets of their intestinal parasites at frequent intervals.