All species of the isopod family Cymothoidae are obligate fish parasites, extracting nourishment through hematophagy and tissue consumption. To elucidate the detrimental effects of this parasitic relationship upon the host fish, we examined body length, weight of body, gonad, liver and stomach contents, and condition factor of Japanese scad Decapterus maruadsi infected with the buccal cavity parasite Ceratothoa carinata in different seasons. During the host fish's breeding season in July, the wet weight and condition factor of male and female host fish ages 1 and 2 were conspicuously diminished. No impacts were detected in September, after the breeding season. We found no impact of the parasite on the stomach content weight or signs of prey fish in the stomachs. Thus, parasite infection with C. carinata potentially diminishes the reproductive success of the host fish by negatively impacting the host's physiological condition, particularly during the breeding season.