Mesoplodon mirus is a toothed whale and can be recognised as such by the single blowhole and the presence of teeth (rather than baleen). It is a member of the beaked whale family with the characteristic V-shaped crease on the throat and the short dorsal fin set relatively far back. True's beaked whale is a small beaked whale that can reach up to 5.3 m in length. The lower jaw has a single pair of teeth (exposed only in adult males). The smooth forehead rises at a shallow angle and is bulging in appearance. It has a distinct beak and the mouthline is curved down at rear. True's beaked whale has a grey dorsal and lateral colouration with a lighter belly and darker areas around the eyes. Adults are often covered with scratches and scars.True's beaked whale may be confused with Sowerby's beaked whale Mesoplodon bidens but can be recognised by its bulging but smooth forehead and its slightly shorter beak. Little is known about the aggregational behaviour of True's beaked whales but it is not expected to differ from similar species such as Mesoplodon bidens (Kinze, 2002).