In order to clarify the seasonal variations of plasma sex steroid hormones and vitellogenin (VTG) concentrations in the wild male Japanese dace, Tribolodon hakonensis, we measured plasma levels of testosterone (T), 11-ketotestosterone (11-KT), estradiol-17β (E₂) and VTG, as well as spermatogenetic stages and gonadosomatic index (GSI). Wild Japanese dace were collected from different sites of the Jinzu River basin (including the Takahara River and the Itachi River). The fish from Toyama Bay were also measured the spermatogenetic stages, GSI and VTG levels. The seasonal variations of the hormone levels were discussed in the relationship with various environmental factors. In landlocked fish of the Takahara River, the plasma concentrations of T and E₂ reached the highest levels in May and June. In the fish collected from the Itachi River, plasma concentrations of T, 11-KT and E₂ reached the highest levels during breeding season of April and May. Sexual maturation, evaluating from GSI and the spermatogenetic stages, proceeded earlier in the fish population at Toyama Bay, and afterward it was followed in the fish population at the Takahara River, in associated with a rise of environmental water temperature at fish captured sites. In the male dace, low but detectable levels of plasma E₂ were measured and there were significantly positive correlations between E₂ level and the levels of GSI, VTG or T. These results suggest that E₂ might be a necessary sex steroid hormone related to gonad maturation, and that circulating E₂ may induce VTG production in the wild male Japanese dace.