Two closely related species, the soybean podworm, Matsumuraeses phaseoli, and the podborer, M. falcana, gives differential economic damages on crops. It is difficult to discriminate these potential sympatric species by morphological characters. The goal of this study was to develop a discriminating molecular marker based on polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). A partial genomic fragment (500 bp) of mitochondrial cytochrome oxidaseⅠ (mtCOI) was sequenced in both species, in which restriction site by Rsa I was selected as a dichotomous marker. PCR-RFLP in the mtCOI region clearly discriminated both species.