To study the relationship and the intrinsic link between the expression of liver GST(glutathione S-transferases) gene in mud carp(Cirrhinus molitorella) and Nile tilapia(Oreochromis niloticus) and the amount of cyanobacteriad,the expression of liver GST gene from the two fishes were analyzed using real-time RT-PCR and the amount of cyanobacteria intake of fish were determined in Xiangang Reservoir in different seasons.The results showed that April was the time that cyanobacteria bloomed,and simultaneously,mud carp and Nile tilapia intake the largest amount of toxic cyanobacteria.The GST genes in mud carp were expressed significantly lower in April than the rest time of the year,which suggested that mud carp may be resistant or tolerant totoxic cyanophyta.Both GSTA and GSTR2 genes in Nile tilapia had the highest expression in April.However,in the other months,the expression of GSTT was significantly high but GSTK was low in mud carp.While GSTA and GSTR2 expressed relatively low in Nile tilapia.It was obvious that GSTT of mud carp played an important role in detoxication process,but as biomarkers for the identification of toxic,it could be used only in low concentration of microcystin.The expression of GSTA and GSTR2 were positively related to the quantity of toxic algae intake in Nile tilapia.Therefore,the expression of GSTA and GSTR2 in Nile tilapia can be used as biomarkers to detect toxic algae.As Nile tilapia can consume large amounts of toxic algae,and its GST gene can detoxifytoxic algae,Nile tilapia was able to be used to biologically control of alga and finally improve water quality.