Parasites are amongst the most diverse and ecologically important animals, having integral roles in maintaining ecosystem health and functioning. In addition to their contribution to biodiversity, they may also have negative impacts on the fauna of the ecosystems they invade. Specifically, parasites co-introduced with their fish hosts, which are recognised as the most common invaders of freshwater ecosystems, have the potential to infect and cause pathologies in native taxa. Although the versatile impacts of non-native fish species have been widely studied, the impacts of their co-introduced parasites have not. To date, at least 47 non-native aquatic species have been introduced into Lithuanian freshwater ecosystems (LFEs) with fish comprising the majority (27 species). However, LFEs have not yet been adequately assessed regarding the damage caused by non-native organisms including the possibly highly deleterious impacts of their associated parasites. Data pertaining to co-introduced parasites which may have spilled over onto native fish and data on non-native fish acquiring parasites following their introduction (spillback) are virtually absent. Moreover, in Lithuania, the fish parasite diversity has not been assessed since the 1980s. Therefore, we proposed a dedicated study to fill this gap and to allow for fish parasitology in Lithuania to be revived. The principal objective of the proposed project is to comprehensively assess the current diversity of helminths (the most common co-introduced group of fish parasites), their abundance, and community structure in native and non-native fishes in selected LFEs and to investigate the changes in native fish helminth communities and their dynamics in response to new introductions.