The presence of the anisakid nematode Hysterothylacium gadi aduncum (Rudolphi, 1802) is reported for the first time in cultured rainbow trout (Oncorhynchus mykiss Walbaum, 1792) as a new host in freshwater farms from Turkey. This marine parasitic nematode is thought to complete the life cycle in fresh water and rainbow trout could act as the definitive host for this parasite under fresh water culture conditions. Mortalities in the rainbow trout infected with H. gadi aduncum (Rudolphi, 1802), were seen after three to four months period of feeding program with marine fish offal (Sprattus sprattus, Engraulis encrasicolus) and pellet fish food. In the present study, the main cause of mortalities was detected as stomach obstruction formed by aggregation of the adult nematodes which developed from infective third stage larvae. Original measurements and figures are presented.