The relationship between quinclorac (3,7-dichloro-8-quinolinecarboxylic acid) tolerance and scavenging capacity of reactive oxygen species (ROS) in Eleusine indica was investigated to understand the tolerance mechanisms of E. indica to the herbicide. E. indica was approximately 104-fold more tolerant to quinclorac than Digitaria adscendens based on GR 40 (herbicide dose required to cause a 40% reduction in plant growth) values determined 6 days after treatment. Quinclorac (10μM) induced the overproduction of ROS (presumably superoxide anion (O 2 − )) in the root tips of D. adscendens 24h after treatment. On the other hand, 10μM quinclorac did not induce the ROS production in the roots of E. indica. The inherent superoxide anion scavenging activity (SOSA) was 4.2-fold higher in E. indica than that in D. adscendens. The constitutive activities of catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11) and glutathione reductase (GR, EC 1.6.4.2) were 1.9, 4.7, and 3.0 times higher, respectively in E. indica than those in D. adscendens. Exogenously applied H 2 O 2 , one of the main ROS, decreased the chlorophyll content in leaf discs of E. indica and D. adscendens. However, the chlorophyll content in E. indica was much higher than that in D. adscendens, indicating that E. indica with its high antioxidant capacity can alleviate H 2 O 2 -induced phytotoxicity and is more tolerant to H 2 O 2 than D. adscendens. These results suggest that the high scavenging capacity of ROS in E. indica could be one factor in its tolerance to quinclorac.