AbstractThe distribution of two acanthocephalan species (Pomphorhynchus laevis and Acanthocephalus anguillae) in the chub (Leuciscus cephalus) was studied in foriver reaches characterized by different levels of pollution: the River Ticino ne Abbiategrasso (unpolluted), the Naviglio Grande Canal, in Milano (slightly polluted), the River Lambro near Merone village (polluted) and the Riv Lambro near Monza (severely polluted).Pomphorhynchus laevis was restricted to the unpolluted and the slight polluted sites, while the intensity of A. anguillae increased proportionally water pollution. These differences were partially explained by the variation in abundance of their intermediate hosts (Echinogammarus stammeri for P. lavis and Asellus aquaticus for A. anguillae). Data on the occurrence of P. laevis at A. anguillae showed a significant negative binomial frequency distributic suggesting their tendency to be aggregated within the host populations L. cephalus.