Abstract: Unit for controlling the shaft for selection and engagement of gears, provided with a control shaft, which can be connected to the said shaft for selection and engagement of the gears, with a device for movement, which can impart to the shaft angular movements which are designed to implement the passage between two angular engagement positions of gears which belong to a single row, and with a cam-guide device, which can modify the said angular displacements, selectively by command, into combined axial and angular displacements, such as to implement the change of gear between consecutive gears which belong to consecutive rows; the cam-guide device being axially mobile along the control shaft, between a plurality of row-change positions, at each of which it can guide the control shaft in the said combined angular and axial displacement, which can implement the passage of gear between two consecutive gears which belong to two consecutive rows; the cam-guide device also being able selectively to use the angular d