Abstract: Beta -cypermethrin is one of the synthetic pyrethroid, which is widely usedto control insect pests andisfound to be more effective than organphosphates and carbamates. However, it is also reported to be more toxic than organphosphates and carbamates. It has been suggested that synthetic pyrethroids are readily absorbed by the gills of fish even at very low concentrations. Glutamic -pyruvic transaminase (GPT)and glutamic -oxaloacetic transaminase(GOT)exist in the cells of every tissue, particularly in the liver. If the liver is damaged, the enzymes are discharged from the cells into the serum.It increases the GPT and GOT metabolic activities quickly. It is, therefore, proposedthat the effects of toxicants on serum GPT and GOT couldbe used as indexes in toxicological assessments. In this experiment, the effectiveness of beta -cypermethrin on the acute toxicity and on serum GPT and GOT concentration of crucian carpwas studied. The LC50of beta -cypermethrin for crucian carp was 11. 4 g/L. The crucian carp serum was treated against beta -cypermethrin levels of 0. 114 g/L, 0. 57 g/L and 1. 14 g/L for exposure times of 0d, 5d, 10d, 15d and 20d, at all concentrations. The results show that when the beta -cypermethrin concentration increases and the exposure time is prolonged, the GPT andGOT activitiesof serum increased also. The resultsproved that beta -cypermethrin could cause great damage to crucian carp and its biochemical functions.