The cell line GCG originated from o Vary of grass carp (Ctenpharyngodon idellus Cur. et Val.) was treated with ethyl methane sulfonate (EMS) for 72 hours. After stable expression of mutation for 3 passages, the cells were treated with 6-mercaptopurine (6-MP) from 4μg/ml to 100μg/ml by stepwise procedures. HaVing examined growth of the isolates in HAT mediums, we obtained mutants named FMR-1 which are deficient in hypoxanthine guanine phosphoribosyl transferase (HGPRT). Identification of biological characteristic of FMR-1 cells showed that chromosomes dec reased in number compared with those of the parental GCG cells. Growth curves of FMR-1 and GCG in TC-100 mediums containing 6-MP (20μg/ml) and no 6-MP were also illustrated. The analysis of LDH and G-6-PD isoenzymes of FMR-1 cells displayed almost the same patterns as those of GCG cells. Further measures to study on the fish HGPRT deficient cells and possible applications of these cells were discussed.