Abstract: The resistant levels of different biotypes of Ammannia arenaria collected from Ningshao plain and Hangjiahu plain within the Yangtze River Delta region to bensulfuron-methyl(BSM) was investigated by agar method.For the 88 biotypes tested,96.6% of them were resistant to BSM,of which the percentage of low resistance (3RI≤10),middle resistance (10RI≤50) and high resistance (RI>50) were 22.7%,53.4% and 20.5%,respectively.The RI of biotype NB0143-01(from Ningbo) and SX077 (from Shaoxing) were as high as 124.4 and 120.4,respectively.Among five regions of Ningbo,Shaoxing,Hangzhou,Jiaxing and Huzhou cities for seeds collection,the biotypes from Shaoxing were the most resistant to BSM with the average RI of 53.1,the biotypes from Ningbo took second place with the average RI of 35.1,and those from Huzhou,Hangzhou and Jiaxing were relatively less resistant to BSM,with the average RI of 24.1,19.9 and 18.7,respectively.The results indicated that the resistant level of A.arenaria to BSM was really high,and the resistance generally occurred in paddy rice growing area of Ningshao plain and Hangjiahu plain.The reason of the higher resistance level of the weed to BSM in Ningshao plain was probably related to the higher selection pressure and continuous use of this herbicide for long time.It's the first report for the distribution of resistant populations of A.arenaria to BSM.