Abstract: The present invention is concerned with an anti-backlash device for a pump shaft comprising an asymmetrical idler (3) pivoting around a fixed axis (4) and an elastic device (5) which produces a restoring force that tends to press the idler (3) against the surface of the pump shaft (1). The anti-backlash device further embodies at least one fin (7) firmly attached to the idler (3) and preferably arranged nearly perpendicular to the direction of flow of the pumped fluid, so that the motion of the fluid will produce a force which, when applied to the fin, yields a moment with respect to the idler axis slightly greater than that produced by the restorative force applied in the opposite sense.