The resistance mechanism of Leptochloa chinensis Nees to propanil was investigated, based on propanil metabolism, aryl acylamidase activity, and chlorophyll fluorescence at the 8 week growth stage of L. chinensis.The concentration of propanil in the leaf and culm extracts of the resistant (R) and susceptible (S) biotypes, as measured by gas chromatography (GC), was found to increase after propanil treatment.The concentration of propanil in the leaf and culm extracts of the S biotype at 72 h was 1.55 and 0.49 mg mL -1 , respectively. However, a lower concentration of propanil was observed in the R biotype, as compared to that in the S biotype.The residue of 3,4-dichloroaniline, as measured by GC, was detected only in the leaf extracts of the R biotype. In contrast, no residue of 3,4-dichloroaniline was observed in the S biotype.The level of aryl acylamidase in the leaf tissue extracts of the R biotype was~140% higher than that in the S biotype. The fluorescence studies showed that propanil inhibited the quantum efficiency of the photosystem II in both the R and S biotypes after 2 h of incubation time. However, when the leaf disks were transferred and incubated in deionized water for 48 h, the quantum efficiency increased in the R biotype but decreased in the S biotype. These results suggest that propanil metabolism, enhanced by aryl acylamidase activity, is the most likely factor contributing towards the mechanism of propanil resistance in L. chinensis plants at the 8 week growth stage.