Abstract
The gonads of rainbow trout ramained in a sexually indifferent condition during the sac-fry period. In the swim-up fry approximately two months after fertilization, about one half of the individuals had gonads showing signs of meiosis. Primary growth of oocytes and lamella formations were subsequently observed in the gonads (ovaries) of about one half of the fingerlings. On the other hand, the gonads of the rest of the fingerlings did not show any signs of meiotic activity, but Sertoli-like cells and lobule and sperm duct formations were observed. Therefore, it may be concluded that rainbow trout belongs to the differentiated type of gonochoristic teleosts.
