The expansible plastic fixing plug consists of a plastic body provided with a slot beginning at the front end of the body and extending longitudinally over a portion of the length of the body in a plane passing longitudinally through the body. The plastic body is provided with a longitudinal bore starting from the rear end for insertion of a fixing screw having an outer diameter. The longitudinal bore has a cylindrical portion having an inner diameter corresponding approximately to the outer diameter of the fixing screw and a narrowing portion ending at the slot. To reduce the insertion resistance for the fixing screw the narrowing portion narrows in a direction at right angles to the plane in which the slot extends and has a width in the plane along its length approximately equal to the inner diameter of the cylindrical portion.