Abstract: A tamper-proof fastener consists of a screw body having a flat-faced head portion and a threaded shank portion having at least one slit extending in a radial direction and located in an end of the shank opposite the head portion, the slit forming at least two legs in the end of the shank. The body defines a central bore extending from the flat face of the head portion to a point intersecting the slit at the end of the shank portion to form a squared-off shoulder in at least one of the legs and terminating in a tapered shoulder in at least one other leg. The tamper-proof screw also includes a headless pin which is sized to fit within the bore so that insertion of the pin into the bore causes an end of the pin to abut the squared-off shoulder and to engage the tapered shoulder to deflect the other leg outwardly. The pin is sized to be flush with the face of the head portion when it abuts the squared-off shoulder to eliminate a protrusion which could be grasped to remove the fastener.