The differential lock valve consists essentially of a bi-directional flow divider (dividing and combining) and a directional valve for optionally bypassing the flow divider. It is intended for use in either open- or closed-loop hydrostatic drives with parallel-connected hydraulic motors. When the lock valve is switched OFF, the inlet flow can divide itself among the mo-tors in any required manner. When the lock valve is switched ON, however, the inlet flow is divided into two pressure compensated portions in accordance with the division ratio of the lock valve. The motors are thus driven at fixed speeds, regardless of their respective loads. This arrangement prevents any hydraulic wheel motor from spinning in conditions of poor traction. A balancing orifice can optionally be arranged between the outlets A and B. This allows some redistribution of flow and prevents un-wanted torque build-up between wheels in these circumstances, and when turning.