In connection to the introduction and spread of non-native species into novel ecosystems, there is currently considerable interest in the role that parasites can play in biological invasions. The project investigated the effect parasites may have on the introduction success of Ponto-Caspian gobies within artificially interconnected European rivers. Relationships between changes in parasite community structure and (i) the connectivity to the source host population, (ii) susceptibility of introduced hosts to local parasites and (iii) the demographic parameters of introduced hosts was tested under natural conditions. The interactions between introduced parasites and phylogenetically and ecologically related host species were investigated by application of advanced epidemiological, molecular and experimental methods. The importance of parasites for the introduction success of gobiid species and the potential threat for the local fish fauna was assessed.