Experiments were carried out on the migration, distribution, longevity and pathogenicity of larvae of Aelurostrongylus abstrusus in Helix aspersa. Snails placed in contact with several hundred thousands of first stage larvae, were examined from one to 24 hours until two years after infection. Larvae were first noted in the snails 4 hours after first contact and it was concluded that the migratory phase lasted for approximately 48 hours. Larvae were restricted to the deeper layers of the sole and to adjacent organs while the greatest number recovered from any snail was 1437. Larvae were at first surrounded by an accumulation of macrophage-like cells; this reaction declined and disappeared until by 12–24 months after infection only a single layered connective tissue capsule separated the parasites from the host tissue; sometimes even that structure was lacking. Larvae isolated from snails up to two years after the initial infection, were able to produce parasitic pneumonia in experimental cats. To study possible pathogenic effects of larvae on snails, a group of the latter were kept in constant contact with first stage larvae. One snail died at 15, three at 17, three at 19 and five at 27 days after initial contact, but despite the different exposure periods there was little variation in the number of larvae recovered from each snail. In addition, only third stage larvae were isolated from the last five survivors, which suggested that there was an immunological response by the snails. The importance of these results is discussed.