An expansion screw for regulating teeth has two expansion-screw bodies, the spacing between which can be changed by means of a spindle, and has at least one guide pin which is aligned parallel to the spindle and engages in guide openings of the two expansion-screw bodies, the spindle having a first and a second threaded portion, each of which is connected to an expansion-screw body, and an actuating portion, with at least one point for application of a tool for rotating the spindle, and the expansion screw has a locking mechanism which interacts with the actuating portion in order to secure the spindle against unintended rotation. In order to develop the expansion screw such that it requires a smaller amount of installation space, it is proposed that the locking mechanism has at least one locking element with a locking member which is disposed laterally alongside the actuating portion and subjects the actuating portion to an axial spring force when the spindle is rotated.