Abstract: A thumb adjustable, parallel jaw end wrench, that uses a worm gear and a movable jaw with a rack, is provided with a locking feature. The worm gear is so mounted in the fixed jaw portion of the wrench that it has play in its axial direction. A first hole ("cam follower hole") that extends perpendicularly away from the head end of the gear is intersected by a second hole ("cam pin hole") running perpendicular to the cam follower hole. Biasing means urge the worm gear toward the cam pin hole. A cylindrical pin having a cam surface with high and low positions that extends radially around a central segment of the pin is rotatably mounted in the cam pin hole. Cam follower means slidably mounted in the cam follower hole ride on the cam surface and extend to the head end of the worm gear. To lock the wrench's grip on an object, the pin is turned to its high cam position, causing the follower means to push the gear, and therefore the rack and movable jaw as well, toward the fixed jaw.