Acilius sulcatus, commonly known as the grooved diving beetle, is a species of aquatic beetle belonging to the family Dytiscidae. It is characterized by its elongated body shape, dark brown or black coloration, and distinct grooves on its elytra (wing covers). These beetles are excellent swimmers and are often found in freshwater habitats such as ponds, lakes, and slow-moving streams. They are predators, feeding on small aquatic invertebrates and even small fish.