First-stage larvae of Angiostrongylus cantonensis from rat feces developed to infective third-stage larvae in 20 of 26 species of freshwater mollusks experimentally exposed. Numbers of larvae up to 2,000 per snail and survival of larvae for a year were observed in Biomphalaria glabrata. First-stage larvae were susceptible to drying but survived at least 3 weeks in freshwater and were infective to snails after at least 2 weeks in either freshwater or seawater. B. glabrata was routinely infected by ingestion of infected rat feces, and first-stage larvae were observed penetrating the wall of the intestine. A cellular reaction enclosed the developing larvae in tissue nodules. Isolated third-stage larvae were susceptible to drying but survived up to 11 days in B. glabrata removed from water, larvae remaining alive 4 days after death of the snails. Rats became infected by drinking water containing third-stage larvae, and larvae were observed active in freshwater for up to a week. Infected B. glabrata died a few minutes after immersion in seawater, but yielded by digestion numerous infective larvae after 4 days in seawater, and a few infective larvae after 5 days. Results suggest the potential importance of freshwater mollusks as intermediate hosts and the possibility of infection of marine hosts. The life cycle of Angiostrongylus cantonensis (Chen) Dougherty, the rat lungworm, was described by Mackerras and Sandars (1955). A. cantonensis has been suspected as the etiologic agent of eosinophilic meningoencephalitis in several Pacific Islands (Rosen et al., 1961, 1962; Horio and Alicata, 1961; Alicata, 1962a, 1962b; and Alicata and Brown, 1962). Human infections were reviewed by Rosen et al. (1962); by Weinstein et al. (1963); and by Kuntz and Myers (1964). Terrestrial mollusks, both slugs and snails, have been infected (Mackerras and Sandars, 1955; Weinstein et al., 1962; and Alicata, 1962a). Experimental infection of the freshwater snail Fossaria ollula was reported by Alicata and Brown (1962). Liat et al. (1965) found three species of freshwater snails infected in nature with A.