Abstract: An automatic hydrostatic transmission in which two vane pumps have their pumping chambers coupled together in a closed circuit with a constant volume of hydraulic fluid, the volume of each pump being changed by the axial displacement of a moving side of the chamber and the respective rotor toward and away from a diaphragm through which the vanes can extend so that either pump can act as the displacement inducer or as the driven member. One of the rotors has a nut provided on a large-pitch screw for automatic adjustment of the pump chamber volume in accordance with load conditions while the other moving side assembly has a key and keyway connection to its shaft.