This paper presents studies of inseination cytology on hybri-dization botween grass carp (Ctenopharyngodon idellus) and freshwater bream (Me-galobraman terninalis). The experiment reveals that the spermatozoon of the bream canether the egg cytoplasm of grass carp, then induces the emergence of the aster andfinally forms the male pronucleus. At the same time, the activated egg begins tocomplete the second polar-body. After the male and female pronulei come into being,they become contacted and fused. The author has ever studied the chromosomes of grass carp, the freshwaterbream and the F₁, hybrid. It has been found that the F₁ hybrid belongs to triploidalthough the parents are diploid. Hence it is gussed that the reason for producingtriploid is to emerge mitosis at the time of fertilization or to reserve seccnd polar-body.