The lesions associated with the migration of the larvæ of Ascaris megalocephala through the tissues of the host were studied in rabbits, guinea pigs and mice. In addition to studying the changes in those organs directly invaded by the larvæ in the course of migration, an organ remote from the migratory route but exposed to the action of any toxic products elaborated by the larvæ was also examined. In each case therefore, sections were prepared from (a) liver, (b) lungs and (c) kidneys.