Abstract: In the catalog,the red crucian carp(Carassius auratus red var.) with 100 chromsomes belongs to the Cyprinidae,Cyprinidae subfamily,Carassius, with karyotype of 22m+34sm+22st+22t;and the common carp(Cyprinus carpio L.) with 100 chromosomes belongs to the Cyprinidae,Cyprinidae subfamily,Cyprinus,with karyotype of 22m+34sm+22st+22t.The previous studies had indicated that the first(F₁)and second generation(F₂)of hybrid fish of red crucian carp(♀)×common carp(♂)were diploid,and F₂ hybrids could produce unreduced eggs and sperms,which mated each other to form fertile allotetraploid hybrid fish(F₃).In order to explore the pathway of polyploidy occurrence in distant crossing of red crucian caip and common carp and the potential of producing unreduced gametes of F₂ hybrids,we studied the chromosome in embryonic cell of F₁ and F₃ hybrid fish.The result showed that the F₁ embryo was diploid,no haploid and polyploidy embryos were observed.While F₃ embryos showed 100,150,200 and even 300 chromosomes,and it was inferred that the chromosomes number in germ cells of F₂ had doubled one or more times,and the F₂ produced diploid and polyploidy gametes,which mated each other to form F₃ embryos with different ploidy.Taken together,it was concluded that F₁ hybrids which come from crossing parents with similar genome size and karyotype,have not displayed polyploidization,while the diploid hybrid progenies could produce unreduced gametes,they fertilized to form polyploidy fish in F₃.The pathway provided important guidance for the study of breeding polyploidy fish by distant crossing.