During 2013 two outbreaks of massive infestation by metacercariae in wild chub Squalius cephalus and farmed black bullhead Ictalurus melas were observed. The metacercariae were about 1 mm, whitish and scattered in muscle. The morphological study allowed identifying the parasites as Hysteromorpha triloba, a cosmopolitan Diplostomidae previously reported in Italy only at the adult stage in Ciconiiformes. Histological examination of skeletal muscle of infected chub revealed a massive presence of metacercariae within large elliptical cavities containing amorphous basophilic substance. At opposite ends of the cavities a mild inflammatory reaction with fibroblasts, lymphocytes, macrophages and rodlet cells was observed. Muscular fibres around the metacercariae presented hyaline degeneration and in some sections appeared replaced by granulation tissue. In black bullhead inflammatory reaction toward H. triloba appeared more severe than observed in chub; metacercariae were enclosed in roundish cysts characterized by a thick wall of granulation tissue, fibroblasts, epithelioid cells and macrophages. The differences observed regarding the tissue reactivity and pathogenic effects of H. triloba metacercariae in chub and black bullhead may be related to a different degree of adaptation of the parasite to autochthonous (the former) and allochthonous (the latter) host. The macroscopic evidence of metacercariae of H. triloba in the muscle, even in the absence of zoonotic implications, could adversely affect the marketability of parasitized fish species.