The paper provides a list of the non-indigenous animal species occurring today in Italian inland waters. Xenodiversity was found to amount to 112 species (64 invertebrates and 48 vertebrates), which contribute for about 2% to the inland-water fauna in Italy. Northern and central regions are most affected, and Asia, North America, and the rest of Europe are the main donor continents. The large majority of non-indigenous species entered Italy as a direct or indirect effect of human intervention. A difference between invertebrates and vertebrates was found for their mode of arrival (unintentional for invertebrates and intentional for vertebrates). Accidental transport, in association with both fish (for aquaculture or stock enhancement) and crops, has been the main vector of invertebrate introductions, whereas vertebrates were mostly released for stocking purposes. Overall stock enhancement (47.92%) and culture (37.5%) prevailed over the other pathways. Seventeen and 7 species of our list are included among the 100 worst invasive species of Europe (DAISIE) and of the world (IUCN), respectively. For some (but not all) non-indigenous species recorded in Italy the multilevel impact exerted on the recipient communities and ecosystems is known, even if rarely quantified, but knowledge on their chronic impact is still missing. Additional research is needed to provide criteria for prioritizing intervention against well established invaders and identify which new potential invader should be targeted as "unwanted"

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In a small stream of southern Tuscany (Fossa Calda), fed by hot springs, we discovered a fish community dominated by tropical species, some of which have never previously been reported in Euro-Mediterranean natural freshwater environments. The aim of our research was to ascertain whether the three most abundant and widespread species (Amatitlania nigrofasciata, Hemichromis sp. and Oreochromis niloticus) have become established. Analysis of size class distribution and growth curves showed that the populations of these species are quite large and made up of juveniles, subadults and adults, as is the case in self-sustaining populations. These fishes were probably released intentionally, since they are widely used in aquariums and aquaculture and their survival in Fossa Calda was possible because of the constant high temperature of thermal waters. Spread to other streams seems unlikely. In fact, at some distance from the hot springs, where water temperatures are lower, populations of the tropical species were small.

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