Abstract: The resistance of 29 Echinochloa crus-galli (barnyardgrass) biotypes to quinclorac collected from the rice planting areas of south China in 2000 and 2001 was determined. The results showed that the biotype collected from Gongdong Huadu was the most susceptible barmyardgrass to quinclorac, the quinclorac IC50 value (dose causing 50% reduction in shoot length) was 0.1480 mg/L, however, the biotypes collected from Hunan Anxiang in 2000 and 2001 were highly resistant to quinclorac, the quinclorac IC50 value was 7.458 and 13.80 mg/L, the resistance ratio was 50.4 and 93.2. The biotype collected from Hunan Changde in 2000 was also highly resistant to quinclorac, the resistance ratio was 4.47. The biotypes collected from Hubei Huangmei, Hunan Hanshou, Hunan Changde in 2001, Hunan Gaoqiao were medium resistant to quinclorac, the resistance ratio was 2.37, 3.12, 2.44, 2.20. The other 21 biotypes were still susceptible to quinclorac.