Abstract: Fish diversity has been paid more and more attentions because of bio-diversity crisis.That how fish diversity is so rich is gradually getting a hotspot in research fields."Chance favors the prepared genome."Present studies suggest that plastic genomes be genetic basis for speciation,form evolution,adaptation etc.In this paper,we tried to give evidences at both chromosomal and molecular levels on why and how fishes evolve in such diverse species.It suggests that fishes have plastic genomes by existences of the variable ranges of chromosome numbers,polyploidy,supernumerary (or B) chromosomes.In addition,gene clusters relevant to diverse forms and other clusters by polyploidy at molecular level have been gradually found.Therefore,chromosomal and molecular characteristics suggest plastic genomes in fishes.Due to plastic genomes,polyploidy and genetic introgression,especially polyploidy,may contribute to species diversity and population genetic polymorphism.Yet unfortunately,plastic genome may be the genetic defect of loss of fish diversity.