Abstract: The cell line GGG originating from the ovary of grass carp(Ctenpharyngodon idellacur.et val)was treated with ethvl methane sulfonate(EM5)for 72 hours.Aner stable expression of mutation for 3 passages, the eells were treated with 6-mercaptopurine(6-MP)from 4μg/ml to 100μg/ml by stepwise procedures.The isolates did not grow in HAT mediums. Mutants, which have been named FMR-1,and which are deficient in hypoxanthineguanine phosphoribosyl tranferase(HGPRT)were obtained.Identification of biological characteristics of FMR-1 cells showed that chromosomesdecreased in number compared with those of the parental GCG cells. The growth curves of FMR-1 ells and GCG cells in TC-199 mediums which contained 6-MP(20μg/ml)and no 6-MP Were also illustrated and compared. The electrophoretogram of G-6-pD isoenzyme of FMR-1 ells displayed almost the same patterns as that of GCG cells.Further measures to study on the fish HGPRT-deficient cells and possible application ofthese cells are discussed.