Primary structural information including the complete nucleotide sequence of the ®rst insect aldehyde oxidase (AO) was obtained from the common house mosquitoCulex quinquefasciatus(Say) through cloning and sequencing of both genomic DNA and cDNA. The deduced amino-acid sequence encodes a 150-kDa protein of 1266 amino-acid residues, which is consistent with the expected monomeric subunit size of AO.