The diamondback moth, Plutella xylostella, is one of the most important pests of cole crops in the world and is the first insect to evolve resistance to Bt toxins in open-field populations. To search for useful molecular markers for Bt reistance monitoring, the PCR-based restriction fragment length polymorphism (RFLP) profiles of three aminopeptidase N (PxAPN1, PxAPN2 and PxAPN4) were determined for 15 representative regional field populations of P. xylostella. Most regional samples had similar RFLP patterns, whereas PxAPN1 from four regions and PxAPN4 from two regions showed different banding patterns after restriction enzyme treatment, but no differences were found in PxAPN2 among populations. The DNA sequence analysis revealed that a point mutation at the restriction site was responsible for the polymorphism of PxAPN1 but no mutations were observed in PxAPN4. Comparing amino acid sequences of PxAPNs from regional populations with reference PxAPNs (GenBank accession no. AAB70755) revealed that four regional populations possessed a point mutation in the Cry1A binding site of PxAPN1 and five regional populations possessed a deletion of eight amino acids in PxAPN4. These RFLP patterns were consistently observed in Southern regions of Korea, including Kyungsangnam-Do and Jeju-Do. The functional association of these RFLP with Bt resistance is currently under investigation