30 ayu ( Plecoglossus altivelis ) samples were collected from the cultural net in Fuxi Town , Ninghai County , Zhejiang Province. Polyarylamide gel electrophoresis was used to detect the expression of isozymes in 8 organsortissues: eye, liver, kidney, muscle, spleen, heart, gill andpectroralfin. ADH,CAT,POD,ACP, ALP,EST,LDH,MDH,ME,GcDH,SCD,GDH,SDH,SOD,ATP were analyzed and the biochemical genetic results showed that 15 isozymes were coded by 55 gene loci ,12 of which (ADH21 ,ADH24 ,CAT22 , POD23 ,ALP2 2 , EST23 , EST24 , LDH25, ME24 , GcDH24 , GDH, SDH22) were found polymorphic. The population of ayu ( Plecoglossus altivelis) in Fuxi town showed the higher genetic diversity when compared with other freshwater fishes,with21.8%oftheproportionofpolymorphiclociand0.0459oftheaverageheterozygosityperloci. And we think the expression of CAT, POD and SOD of ayu ( Plecoglossus altivelis ) can be used to evaluate the influence to body2protecting system of the fish when environmental factors were changed. The expression of isozymes in pectroral fin can be used as genetic marker in breeding of ayu ( Plecoglossus altivelis).