Abstract: A lock mechanism for a rotatable stub such as the operating shaft of a fluid flow control valve has a main body 10 connected to a drive member 12 adapted for mounting on the rotatable stub. A locking sleeve 17 is adapted to fit closely to the machine (e.g. valve) on which the mechanism is fitted so as to be held against rotation, and mounted in the body are two key-operated lock members 25 engageable with the sleeve 17 to lock the body against rotation. Associated with each lock member 25 is a control plate 56, movement of which can be blocked by a stop member 53 so preventing operation of the associated lock member 25. The two stop members 53 are threaded on studs 55 fixed in the body 10, and are moved linearly on being rotated by respective toothed wheels 46, driven by a peg 62 on sleeve 17 as the body 10 is rotated.