Red crucian carp(abbreviated as RCC)with 100 chromosomes belonged to the Cyprininae subfamily,and the blunt snout bream(Megalobrama amblycephala)(abbreviated as BSB)with 48 chromosomes belonged to the Cultrinae subfamily.In the first generation of RCC(♀)×BSB(♂),we obtained triploid hybrids(abbreviated as 3nRB)and tetraploid hybrids(abbreviated as 4nRB).In this study,we made the analysis of karyotype of the chromosomes,FISH of the chromosomes and the gonadal development in 3nRB.The results are as follows:(1)3nRB hybrids had 124 chromosomes with a karyotype of 31m+45sm+26st+22t,which possessed two sets of RCC-derived chromosomes,and one set of BSB-derived chromosomes.(2)The species-specific repetitive sequences probe hybridized to 100 chromosomes in RCC but none in BSB.In 3nRB,the probe hybridized to 100 chromosomes,suggesting that they possessed two sets of RCC-derived chromosomes.(3)The gonad of 3nRB displayed developmental anomaly,which was considered as a retrogression trait.This paper discussed the formation mechanism of 3nRB,and provided experimental data for study of distant hybridization and polyploidy in fish,which is of importance in genetic breeding.