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To investigate the potential effects of female presence upon serum levels of vitellogenin (Vg) in male fish, we held male Japanese whiting Sillago japonica in the presence (Group FM) or absence (Group M) of females for three weeks. Fish were first reared for three weeks after the grouping and then sampled from each group at four-hourly intervals. Analyses were performed on serum vitellogenin levels, gonad somatic index (GSI: gonad weight /body weight x 100), and testicular histology. Serum levels of Vg in males from Group FM were higher than those of males from Group M. Moreover, GSI of Group FM males was also higher than males in Group M, although histological analysis demonstrated active spermatogenesis in the testes of males from both groups. These results suggest that the presence of females causes an increase in the serum levels of Vg and GSI in male fish.