A study has been made on the gonadal development and histological changes of thehypophysis in the mullet (Mugil soiuy Basilewsky) from three environments differingin salinity: seawater, freshwater ponds, and oligohalinous-water ponds (salinity2‰).The results of experiments on the induced spawning of mullet reared in freshwater andoligohalinous-water ponds are given, and some measures which might be effective ininduced spawing are suggested. Environmental salinity effects the relative size of the RPD (rostral pars distalis)and the PPD (proximal pars distalis). And the increase in size of the RPD is accompaniedby an decrease in size of the PPD. In mullets collected from freshwater and oligoha-linous-water ponds, the RPD is 61. 3% larger than that of fish collected from seawater.And the activity of prolactin-secreting cells is also stronger. On the other hand, the PPDof the former is 45. 8% smaller than the latter, and the gonadotrops are less active, asdemonstrated by staining reaction. The development of the oocytes of freshwater or oligohalinous water pond culturedmullets does not go beyond a point in between the early and the middle stage of phase IV. Only a few oocytes can develop to the middle stage (mean oocytes dismeter 780μ). Mostmullets in such a state failed to induce spawning. Only two females were successful ininducing spawning from which we obtained 34 and 17 larvae respectively that lived 17-48 houres. Male mullets cultured in freshwater or oligohalinous-water ponds may reachsexual maturity.