AbstractFour possible biochemical mechanisms of organophosphate resistance were examined in populations ofRhyzopertha dominica(Fabricius) collected from eight locations in the states of Minas Gerais and São Paulo in Brazil and from seven locations in northeast Kansas, United States. Results from insecticide bioassays with synergists (triphenyl phosphate, diethylmaleate, and piperonyl butoxide) suggested that metabolic detoxification mechanisms might be involved in organophosphate resistance in some of the populations ofR. dominica. However, overall low synergism, non-synergism or even antagonism presented by these synergists in all 15 resistant populations suggested that carboxylesterases, cytochrome P450 monooxygenases and glutathioneS-transferases were not likely to play a major role conferring organophosphate resistance inR. dominica. This hypothesis was strongly supported by our biochemical studies showing no significant difference in these enzyme activities, as well as cytochrome b₅, P420, and P450 levels between the susceptible and resistant populations. In contrast, all resistant populations ofR. dominicashowed higher acetylcholinesterase activity than the susceptible population both in the presence and absence of malaoxon. Furthermore, acetylcholinesterase from the resistant populations was less sensitive to inhibition by malaoxon than that from the susceptible. These results suggested that quantitative and qualitative changes in acetylcholinesterase might contribute to organophosphate resistance in these populations ofR. dominica.