We simultaneously investigated the effects of Polycyclic Aromatic Hydrocarbons (PAHs) and a common acanthocephalan intestinal parasite (Pomphorhynchus sp.) on the gut microbial flora of a freshwater fish, the European chub (Squalius cephalus). Individuals naturally infected or not with the parasite were exposed to PAHs at concentrations similar to those previously measured in wild populations from an urbanized watershed.