Abstract: An adjustable open-end wrench is formed with a C-shaped base jaw member having an upper base grip-forming portion and a lower tongue-forming portion which is pivotally connected to an elongated handle. An L-shaped, sliding jaw member has a horizontal leg which is slidably guided between the upper and lower portions of the base jaw member. The sliding jaw member has an upright leg forming a movable grip portion opposing the base grip portion. Corresponding, normally interengaging, tooth-like locking formations are formed on the upper end of the handle and the lower edge of the movable jaw member horizontal leg for normally locking the jaw members relative to each other. The handle may be pivoted to temporarily release the interengaging locking formations so that the movable jaw member may be slidably moved relative to the base jaw member for adjusting the distance between the grip portions to accommodate to different size bolts and nuts.