Hydaticus grammicus (Germer, 1827) (Coleoptera: Dytiscidae) is a common diving beetle with a body length of 9.0–11.0 mm and yellowish brown with vertical stripes on their elytra. We found an unidentified beetle resembling H. rhantoides Sharp, 1882 because of dark brown on its body color in Hyogo Prefecture, Japan. Based on the phylogenetic analysis using mitochondrial COI gene sequences (646 bp), we concluded that the unidentified beetle is a color variation of H. grammicus (no stripe type). This color variation with no stripe is the first report of this species.