Attacin is an antibacterial protein that is secreted by fat body cells of insect larva in response to bacterial infection. A 949 bp cDNA encoding the antibacterial protein attacin was isolated by employing annealing control primer (ACP)-based GeneFishing polymerase chain reaction (PCR) from immunized Papilio xuthus larvae. The attacin cDNAs encoded 250 amino acid residues open reading frame with 60 residues prepropeptide. The deduced amino acid sequence of P. xuthus attacin showed significant identities with other Lepidopteran attacins. The attacin transcript was detected at significant level after injection with bacterial lipopolysaccharide (LPS). The predicted mature attacin was expressed as soluble fusion protein efficiently in bacterial expression system. To increase productivity and solubility, attacin was translationally fused with thioredoxin (Trx) protein and expressed in E. coli cells that are highly sensitive to the mature attacin. The recombinant attacin exhibited antibacterial activity against Gram-negative bacteria.