This study assessed the effects of Hexamita salmonis (Moore) on metabolism of rainbow trout Oncorhynchus mykiss (Walbaum) and its effect on the host's susceptibility to infectious pancreatic necrosis virus (IPNV) after antiparasitic treatment. Rainbow trout naturally infected with H.salmonis were treated with 10mg metronidazole kg fish(-1) per day, and their physiological recovery was assessed through measuring resting metabolism on the 7th, 14th, 21st and 28th day after treatment. In addition, we exposed the naive fish to H.salmonis and measured the resting metabolism (oxygen consumption as mg O(2)kg(-1) per hour) on the 10th, 20th and 30th day after the exposure to assess the variation in metabolic rates after infection. Significantly lower rates of metabolic activity (P<0.05) were anticipated 20days after infection with H.salmonis compared with the fish infected with H.salmonis for 10days or with the parasite-free fish. Similarly, the treated fish needed about 20days to fully recover from hexamitiasis. The susceptibility of rainbow trout to IPNV remained unchanged in the presence of H.salmonis. Weight loss was significantly higher (P<0.05) in infected than that in the parasite-free fish. Fish should be examined regularly for H.salmonis and treated immediately whether found to prevent economic losses and excessive size variation.