A guidance system for maintaining a lateral move irrigation machine along its intended path of movement. The center support tower of the machine carries a guide arm which is pivotal relative to the tower about a vertical axis. A guide wheel on the arm travels in a furrow which is formed in the field to extend in the intended direction of travel of the machine. The guide wheel remains in the furrow so that if the machine departs from the desired path, relative rotation occurs between the tower and guide arm. A switch actuator is then pivoted in a direction to open a switch which deenergizes the drive motor of the end support tower which is furthest ahead.