Dytiscus marginalis are the diving beetles with their wings covered by hard wing cases known as elytra. It is the largest order including more species known to science than any other order not only in the class insect, but also in the entire animal kingdom. The order is huge and includes a wide variety of groups. In the present study we have isolated, PCR amplified and deciphered the partial nucleotide sequence of the mitochondrial cytochrome oxidase subunit I gene (COI) of D. marginalis from Kerala and its phylogenetic status. DNA sequence similarity searches of COI gene of D. marginalis (GenBankAcession: KM 230115) revealed that it is genetically identical to Cybister ventralis (DQ 813688) and Cybister cognatus (DQ 813672) isolated from USA. The results indicate slow evolution of the COI sequences among the morphologically distinct and geographically isolated D. marginalis.