The Karyotype of Claris fuscus has been determined from Kidney cell by the method of PHA pericardial Cavity injection in vivo. Slides were made by thehypotoic air drying technique with Giemse staining. The diploid number of Chromoso-mes is 56 (that is 2N=56). The total chromosome arm count is 98. They may be clas-sified into three groups: 9 pairs are metacentrie, 12 pairs submetacentric, and 7 pairssubtelo-or telocentric Chromosomes. The resulte are presented in table 1 and Fig 1. It was found that No. 21 was a pairs of homomorphic chromosomes in the fema-le, but it was a pair of heteromorphic chromosomes in the male, therefore the hetero-morphic chromosomes wore sex chrosmes of C. fuscus in all probabillity. They weret?ely designated as XY. The problem of the heteromorphic chromosomes inPisces was thus discussed, but whether the heteromorphic chromosomes were the sexchromosomes or not awaiting for further investigation.