Abstract: In order to study the mechanism of the biotype of Galium aparine resistant to tribenuron-methyl,the differences on ALS,GSTs,SOD and POD of the resistant (R) and susceptible (S) biotypes to tribenuron-methyl were preliminarily determined.There was no significant difference between the sensitivity of ALS in vitro from the R and S biotypes to tribenuron-methyl,with the IC50,0.682 and 0.718 μg(a.i.)/L,respectively.The ALS activity in vivo of both R and S biotypes increased at the first 2 days after application of tribenuron-methyl,however,the ALS activity in vivo of S biotype increased much less and declined much more quickly,less than the control value 3 days after treatment and lasted out,while that of R biotype is 4.10 times of the control,and declined slowly to the control value 5 days later and kept on.Meanwhile,the GSTs activity of R biotype increased immediately, with the peak value 2.40 times of the contemporary control,while that of S biotype declined at first and then increased,with peak value 1.61 times.Both of them came down to the control value 10 days later.The SOD activity of R biotype maintained the control level,while that of S biotype declined a little,but no significant difference existed.The POD activity of both biotypes increased greatly,but no significant difference existed either.The results indicate that the low level resistance to tribenuron-methyl in the biotype of Galium aparine from Xuchang,Henan Province,is based on the excessive expression of ALS and the enhancement of tribenuron-methyl metabolism resulting from GSTs,not due to the sensitivity of ALS.In the meanwhile POD and SOD may play a role in alleviating the spray injury.