When a species is introduced in a new territory its parasites may also follow, thus forming a sort of biotic unit. We propose the term symbiota to indicate this host-parasite complex. This concept is at the basis of the present research in which 11 species of alien and native freshwater fish were collected by electrofishing in the Po river area (Northern Italy). The alien host fish Ictalurus melas and Silurus glanis were parasitized by only one monogenean species each. Lepomis gibbosus, of North American origin, hosted four monogeneans and Carassius carassius three. All these parasites should be considered as alien species, except those collected from C. carassius that should be considered in part natives and in part aliens. © Springer 2005.