Sixteen species of both native and alien fish were collected and examined for monogenoids from September 2001 to March 2004 in bodies of water located within fishing areas of the Po River (northern Italy). A checklist of monogenoids was compiled and integrated with literature reports of Italian monogenoids, updating the number of known Italian freshwater monogenoids to 35, with 15 new host or locality records (or both). Parasitological data of monogenoids on native and alien demonstrate the ability of alien monogenoids to colonize native hosts. The process of invasion of nonnative monogenoids in Italy started about 30 yr ago and is still ongoing: 17 of the 35 monogenoids now present in Italian freshwaters are considered alien (5 species of monogenoids arrived in Italy from America, 11 from Central Europe, and 1 from Eurasia), 2 are considered native, and the remaining 16 are considered of unknown origin because of our limited knowledge concerning the original distribution of monogenoids.