Abstract: A tamper-proof locking system for a standard fire hydrant, one embodiment of which includes a cap assembly that fits over the valve stem of the hydrant and is fastened thereto. The cap assembly cannot be turned to open the valve except by means of a special wrench which is adjustable to securely grip the assembly. In another embodiment, the system includes a cover assembly which replaces a standard hydrant port cover and cannot be taken off the nipple of the port except by the special wrench.