A hydrostatic transmission includes a hydraulically controlled speed differential means drivingly connected with an output shaft for transmitting the rotation torque of an input shaft to the output shaft and including a hydraulic element functionable as a reaction element therefor, a hydraulic positive displacement pump-motor drivingly connected with the input shaft for controlling the hydraulic element in accordance with the displacement ratio thereof, and a line circuit to provide hydraulic communication between the hydraulic element and the pump-motor. In the hydrostatic transmission, a drive power train selector gear unit is disposed between the input shaft and the speed differential means to selectively complete the forward and reverse drive power trains inbetween the input shaft and the output shaft by way of the speed differential means.