The sequence of acetylcholinesterase (AChE) cDNA from the diamondback moth (DBM), Plutella xylostella (L.) was determined. A 2.3 kb partial cDNA sequence, containing an ORF encoding AChE precursor with 638 amino acid residues was obtained by primer walking. The deduced protein sequence showed the highest homology to AChE of the Colorado potato beetle (CPB) and the green rice leafhopper. The DBM AChE belongs to the Drosophila melanogaster Ace-orthologous insect AChE subfamily. A single AChE gene transcript (7.6 kb) was detected by Northern blot analysis. This is the first report of the complete coding sequence of AChE cDNA from a lepidopteran insect.