Sulfonylurea (SU)-resistant monochoria has recently been found in rice paddies in Korea. A quick and accurate means of confirming herbicide resistance is necessary to take timely management decisions. This article describes a rapid and reliable assay to detect SU-resistant biotype of monochoria. The techniques tested include seed germination, in vivo and in vitro acetolactate synthase (ALS; EC 126.96.36.199) activity, leaf, and whole-plant bioassays. In the whole-plant bioassay, shoot dry weight of the resistant (R) biotype was 3,200-fold less affected by imazosulfuron and sevenfold less affected by pyrazosulfuron-ethyl than the susceptible (S) biotype. Although the whole-plant bioassay is reliable, it is expensive, requires a lot of infrastructure, and takes a few months to complete. The germination rate of the R biotype in petri dish bioassays was > 200-fold less inhibited by imazosulfuron and 100-fold less inhibited by pyrazosulfuron-ethyl than that of the S biotype. Seed germination bioassays in petri dishes do not require as much infrastructure as whole-plant bioassays do and can be completed in a shorter time. Leaf bioassays showed that leaf color of the R biotype was > 1,600- and 300-fold less affected by imazosulfuron and pyrazosulfuron-ethyl, respectively, compared with that of the S biotype. This assay takes about 6 d to complete. In vivo ALS assays showed lower levels of resistance to ALS herbicides than did in vitro ALS assays, where the R biotype was about 200- and 30-fold less sensitive to imazosulfuron and pyrazosulfuron-ethyl, respectively, than the S biotype. All assays successfully distinguished the R from the S biotype, but in vitro ALS assays are the simplest and the quickest. The in vitro ALS assay was chosen as the standard procedure for future confirmation of resistance in monochoria populations. Caution is needed because the in vitro assay is not appropriate in cases wherein the resistance mechanism is increased metabolism of the herbicide or overexpression of the target enzyme. Results should be interpreted in relation to field history and field observations. Follow-up studies also are needed to verify that other resistance mechanisms do not confound the in vitro assay.