A natural population of Deroceras laeve was examined regularly for the presence of Cosmocercoides dukae (Holl, 1928) Travassos, 1931. Transmission of C. dukae in D. laeve occurred in late spring and early summer before adult slugs from the previous year died in July. Transmission to juvenile slugs (hatched from eggs laid in spring) continued in late summer and fall. Histological sections of slugs exposed experimentally to larvae of C. dukae revealed that larvae penetrate slug tissues. Arion fasciatus, Arion hortensis, and Zonitoides nitidus represent new host records for C. dukae.