Abstract: A self-priming centrifugal pump includes a cylindrical metal plate case forming a pressure chamber and provided with a suction inlet and a pressure outlet. The pressure chamber has a bladed impeller with a peripherally arranged diffuser, an axial ejector device including a venturi tube, and a torus-shaped interspace adapted to convey the emitted flow from the radial diffuser towards the pressure chamber. A single curved blade is arranged in the lower portion of the interspace and extends from the peripheral region of the radial diffuser up to a central annular outlet passage of the interspace which extends all around said ejector device in such a manner to counteract the rotational component of the fluid speed direction and to promote separation of air.