Abstract: A tamper-proof cover for the operating nut of a hydrant. The cover includes a housing adapted to be positioned above the operating nut for covering the operating nut, the housing having an opening, a housing mounting device securable to the hydrant and engagable with the housing for securing the housing to the hydrant, and a locking device for removably securing the housing to the mounting device which includes a pin positionable in the opening in the housing and engagable with a recess defined in the mounting device, a locking member engagable with the pin and the recess in the housing, a frangible shoulder within the recess in the mounting device against which the locking member seats and which must be broken to remove the pin. Other forms of the tamper-proof cover and a tool for removing the cover are also disclosed.