The cytochrome c oxidase subunit I (COI) gene region of mitochondrial DNA (mtDNA) was examined in four abalone species to estimate its utility as a genetic marker using restriction fragment length polymorphism (RFLP) analysis. The utility was evaluated in terms of genetic divergence and relationships among Haliotis discus hannai, H. rufescens, H. rubra, and H. midae in both hemispheres of the world. There was clear genetic divergence in the mtDNA COI region between all pairs of the four species. Moreover, relationships among the abalone species were reflected in their geographical distributions and morphological characteristics. Therefore, RFLP analysis of the mtDNA COI region is a suitable genetic marker for the estimation of genetic divergence and relationships among abalone species. However, it is not effective for the evaluation of genetic differences within abalone species.