The occurrence of two acetylcholinesterases, AChE1 and AChE2, in the mosquito Culex pipiens has been recently documented, Resistance to organophosphates and carbamates due to target insensitivity is the result of a qualitative change of only AChE1, encoded by the Ace.1 gene, Because AChE1 and AChE2 differ in their sensitivity to inhibitors, Ace.1 genotypes can be misclassified by previous tests. We describe a new rapid microplate Lest that allows unambiguous identification of Ace.1 genotypes. This test involves comparing AChE activities in the absence of insecticide and in the presence of two propoxur concentrations: a low concentration that inhibits only the sensitive AChE1 and a higher concentration that inhibits also AChE2 but not the insensitive AChE1 responsible of insecticide resistance. This comparison allows the identification of the three Ace.1 genotypes: resistant (Ace.1(RR)), susceptible (Ace.1(SS)) homozygotes, and heterozygotes (Ace.1(RS)), The similarity of propoxur sensitivity of modified AChE1s found in various resistant strains From the United States, Europe, and Africa indicates that this test is probably suitable for all the Ace.1 alleles described so far in C. pipiens.