Herbicidal activity and acetolactate synthase (ALS) inhibition of sulfonylurea derivatives with a fused heterocyclic moiety bonded to a sulfonyl group were investigated. Some compounds that had an imidazo[1,2-b]pyridazine moiety substituted at the 2-position by chlorine or methyl controlled sulfonylurea-resistant (SU-R) weeds and showed inhibitory activity to ALS prepared from SU-R Schoenoplectus juncoides shoot. There was a correlation between in vitro and whole-plant herbicidal activity of the compounds mentioned above against SU-R Schoenoplectus juncoides. Among them 1-(2-chloro-6-propylimidazo[1,2-b]pyridazin-3-ylsulfonyl)-3-(4,6-dimethoxypyrimidin-2-yl)urea, propyrisulfuron, was selected for further evaluation. Propyrisulfuron effectively controlled paddy weeds at doses of 70 and 140 g a.i. ha−1 with good rice selectivity in a field trial.