Abstract: A progressive cavity pump of the type having a stator member and a rotor member disposed within a cavity in the stator member and being rotated while being permitted to orbit about a central axis of the stator in operation, one of the members being formed of resilient material and the other of rigid material. The rotor is of helical formation having a constant circular transverse cross-section and defining a single-start thread of a preselected pitch and direction, and the cavity of the stator is in the form of a two start thread of the same direction as the rotor and twice the pitch, the cavity in transverse cross-section has an outline defined by a pair of spaced semi-circular concave ends and a pair of sides joining the semi-circular ends. The semi-circular ends have a diameter slightly less than the diameter of the cross-section of the rotor for establishing an interference fit between the rigid and resilient members.