[Objective] The aim was to explore the factors affecting the herbicidal activity of pyrazosulfuron-ethyl against Sagittaria pygmaea in paddy soil.[Method] The bioassay method was carried out under greenhouse conditions.[Result] The reduction of more than 80% in fresh weight was observed in S.pygmaea treated with 45 g/hm2 of pyrazosulfuron-ethyl for 7 d or more.Application of 60 g/hm2 at the sprouting stage of S.pygmaea suppressed completely the growth in fresh weight irrespective of tuber size.However,at 6 g/hm2 growth of the weed sustained less reduction with increasing tuber size.Differences in flooding water depth and temperature had no appreciable influence on the herbicidal activity.Growth inhibition of S.pygmaea by pre-emergence or 1.5-leaf stage applications under non-leaching conditions was significantly higher than that under water leaching conditions.Under non-leaching condition,there was no remarkable difference in this inhibition between preemergence and 1.5-leaf stage applications,while under water leaching conditions the growth reduction with the latter application was significantly larger.Growth inhibition of S.pygmaea planted at a depth of 2 cm was significantly higher than that planted deeper （5 cm） with the pre-emergence applications.[Conclusion] This study had provide basis for the further study on the inhibition effect of pyrazosulfuron-ethyl.
