A biotype of Monochoria vaginalis resistant to sulfonylurea herbicides has been identified in paddy fields in Chonnam province, Korea since 1997. The fields have been used for monoculture rice production and have been treated with sulfonylurea herbicide-based mixtures for 8 consecutive years. Five different sulfonylurea herbicides, azimsulfuron, bensulfuron-methyl, cinosulfuron, cyclosulfamuron, and pyrazosulfuron-ethyl with 1/2, 1, and 2 times dosages of the recommended rate of the respective herbicide had no controlling effects on the resistant biotype, suggesting that the biotype has cross resistance to sulfonylurea herbicides. The susceptible and the resistant biotypes exhibited their I_(50) values of 2.5 and 40g ai/ha for pyrazosulfuron-ethyl, respectively, and of 4.5 and 300g ai/ha for LGC-42153, respectively. Other herbicides having different mechanisms of action from sulfonylurea herbicides, such as butachlor, mefenacet, molinate, pyrazolate, and thiobencarb had excellent controlling effects on the resistant biotype when they were applied at 1 or 2 leaf stage. These results imply that the biotype does not have multiple resistance to those herbicides. The resistant biotype of M. vaginalis survived from the paddy fields treated with sulfonylurea herbicide-based mixtures, such as pyrazosulfuron-ethyl/molinate and bensulfuron-ethyl/molinate, could effectively be controlled by soil application of the mixtures of butachlor/pyrazolate or by foliar application of the mixtures of bentazone/2,4-D.