Objective To study the effects of herbicide LeCaoLong on DNA damage of red crucian carp lymphocyte in vivo . Method Single cell gel electrophoresis (SCGE) assay was used to detect DNA single strands breaks (DNA SSBs) when red crucian carps were exposed to different concentration of LeCaoLong. Results At the concentration of 0 88, 1 75, 3 50, 7 00 mg/L, LeCaoLong could induce DNA damage in the peripheral blood lymphocyte of red curcian carp, the comet cell tail length of every group exposed to LeCaoLong was longer than the control group( P<0 05 ). The DNA damage was signficantly positively correlated with concentrations in the range of 0-7 0 mg/L,( r=0 982,P<0 01 ), when red crucian carps exposed to LeCaoLong for 12 h,24 h,48 h,96 h and 10days. The comet cell rates of every concentration group were higher than relative controls. Conclusion All the results suggest that the herbicide can cause significant genetic damage to the lymphocyte of red crucian carp at certain concentration and time.