The gonadosomatic indexes (GSI) of the triploid crucian carp , produced by crossing the alloteraploid hybrids ( ♂) of red crucian carp ( Carassius auratus red var. ) ×common carp ( Cyprinus carpio L . ) with the diploid Japanese crucian carp ( ♀)( Carassius auratus cuvieri T. et S. ) , and the diploid Japanese crucian carp were compared. The indexes of the ovaries , testes and the fat tissue“gonad”found in the triploid crucian carp were respectively lower than those of the ovaries , testes and the average value of the ovaries and testes found in the Japanese crucian carp. The indexes of the ovaries and the testes in the Japanese crucian carp were respectively 2.85 and 1.94 times of the indexes of the ovaries and the testes in the triploid crucian carp. The index of the average value of the ovaries and testes in the Japanese crucian carp was 5. 60 times of that of the fat tissue found in the triploid crucian carp. In the triploid crucian carp , the fish with the fat tissue“gonad”grew faster than the others. The females grew faster than the males. No germ cells in the fat tissue “gonad ”were observed , suggesting that the development of the germ cells was completely repressed. The repressed degree of the ovaries in the female triploids was higher than that of the testes in the male triploids due to the absence of the yolk in the oogonium2like cells that occupy most space of the ovaries. Considering the sterility and the fast growth rate , the triploid fish with the fat tissue“gonad”were the best ones .