ABSTRACT: Over the past decades, as a result of various human activities involving intentional or unintentional movement of animals, many helminth species have been introduced to new regions with several ecological and epidemiological implications for the native species. A high prevalence of infection with an introduced digenean Renifer aniarum, previously known only from North America, was found in the grass snake Natrix natrix in the Calabria region, southern Italy. Morphological and molecular comparison with North American R. aniarum has confirmed the identity of the Italian specimens. A total of 41 grass snakes were studied for R. aniarum infection. Of 24 snakes sampled be­tween 2009 and 2010, 22 were positive for this parasite. In contrast, all 17 snakes sampled from museum collections between 1983 and 1994 were negative. Our results support the hypothesis that R. aniarum was perhaps introduced into this area during the 1990s by the translocation of the American bullfrog Lithobates (Rana) catesbeianus, a normal second intermediate host of the digenean in its native range in North America. Although the life cycle of R. aniarum is complex and includes 3 host stages, this parasite has found suitable first and second intermediate hosts as well as definitive hosts in Italy. Renifer aniarum was second only to the very common grass snake tapeworm Ophiotaenia europaea in both prevalence and abundance among 9 species of helminths recovered in our study.