Inferring parameters related to the aggregation pattern of parasites and to their dispersal propensity is important for predicting their ecological consequences and evolutionary potential. Nonetheless, it is notoriously difficult to infer these parameters from wildlife parasites given the difficulty in tracking these organisms. Molecular-based inferences constitute a promising approach that has yet rarely been applied in the wild.Here, we combinedseveral population genetic analyses including sibship reconstruction to documentthe genetic structure, patterns of sibship aggregation and the dispersal dynamics of a non-native parasite of fish, the freshwater copepod ectoparasiteTracheliastespolycolpus. We collected parasites according to a hierarchical sampling design,withthe sampling of all parasites from all host individualscapturedineight sites spread along an upstream-downstream river gradient. Individual multilocus genotypes were obtained from 14 microsatellite markers, and used to assign parasites to full-sib families and to investigate the genetic structure of T.polycolpus among both hosts and sampling sites. The distribution of full-sibs obtainedamong the sampling sites was used to estimate individual dispersal distances within families. Our results showed that T. polycolpus sibs tend to be aggregated within sites but not withinhost individuals. We detected important upstream-to-downstream dispersal events of T.polycolpusbetween sites (modal distance: 25.4 km; 95% CI [22.9, 27.7]), becoming scarcer as the geographic distance from their family core location increases. Such a dispersal pattern likely contributes to the strong isolation-by-distance observed at the river scale. We also detected some downstream-to-upstream dispersal events (modal distance: 2.6 km; 95% CI [2.2-23.3]) that likely result from movementsof infected hosts.Within each site, the dispersal of free-living infective larvae among hosts likely contributes to increasing genetic diversity on hosts, possibly fostering the evolutionary potential of T. polycolpus.