A rotorcraft provided on top with a fairing (3) arranged below the rotary wing of a main rotor of substantially vertical axis of the rotorcraft, and covering a mechanism enabling the blades of the rotary wing of the main rotor to be operated. The streamlined shape of the fairing (3) is shaped longitudinally like a drop of water that is truncated in its rear portion. The truncated portion (6) of the waterdrop-shaped profile of the fairing (3) forms the rear wall of the fairing (3) and is provided with a transversely middle hump (7). Said hump (7) is provided with projections in relief (9) arranged as waves in a longitudinal succession extending along the side walls of the fairing (3), and the truncated portion (6) is arranged in a plane sloping towards the rear of the rotorcraft.