The activity and sensitivity of acetylcholinesterase (AChE, EC 1.1.1.7) from an organophosphate-susceptible (OSS) and three resistant (OR-0, OR-1 and OR-2) strains of the greenbug (Schizaphis graminum) were biochemically compared. All three resistant strains displayed higher frequencies of individuals with respect to both increased activity of AChE toward the model substrate acetylthiocholine (ATC) and reduced sensitivity of AChE to inhibition by paraoxon as compared with the OSS strain. Kinetic study indicated that AChE from the OR-0, OR-1 and OR-2 strains had 3.3-, 2.7-and 2.3-fold, respectively, lower affinity, but 1.5-, 2.2-and 2.0-fold, respectively, higher catalytic activity toward ATC than AChE from the OSS strain. SigniΓΌcantly increased activity of AChE in the resistant strains was also conΓΌrmed by non-denaturing polyacrylamide gel electrophoresis, and appeared to be associated with the increase of general esterase activity. Inhibition kinetics revealed that AChE from the OR-0, OR-1 and OR-2 strains was 2.1-, 2.2-and 2.7-fold less sensitive to inhibition by paraoxon than that from the OSS strain. The study suggested that both qualitative and quantitative modiΓΌcations of AChE had evolved in the resistant strains and were likely to signiΓΌcantly enhance the overall resistance level in greenbugs.