Cecropin is an antimicrobial peptide that is synthesized in fat body cells and hemocytes of insect in response to a hypodermic injury or bacterial infection. A 503 bp cDNA encoding a cecropin-like antimicrobial peptide was isolated by employing annealing control primer (ACP)-based differential display PCR and 5'-RACE from immunized Papilio xuthus larvae. The open reading frame (ORF) of isolated cDNA encoded a 63 amino acid prepropeptide with a putative 22-residue signal peptide, a 3-residue propeptide and a 38-residue mature peptide with a theoretical mass of 4060.89 Da. The deduced amino acid sequence of peptide showed significant identities with other Lepidopteran cecropins. This peptide was named as papiliocin. RT-PCR revealed that the papiliocin transcript was detected at significant level after injection with bacterial lipopolysaccharide (LPS). Based on the deduced amino acid sequence of papiliocin, a 38-mer mature peptide was chemically synthesized by Fmoc method, and analyzed antimicrobial activity. The synthetic papiliocin peptide had a broad spectrum of activity against fungi, Gram-positive and negative bacteria, and also showed no hemolytic activity against human red blood cell.