The increasing use of hydrostatic drives in industry has motivated development of fluidic control devices, using hydraulic oil, for application to hydrostatic transmission pump displacement control. A closed-loop positioning servo using proportional hydrofluidic amplifiers and flowing hydraulic fluid to replace the numerous mechanical parts common to conventional servos has been developed. This hydrofluidic servo has been built in both a packaged servoactuator form and a modular form for integration with existing hydrostatic transmission pumps. Operating principles and performance of experimental units are discussed. Operation was evaluated over a 160-500 psig (11-34.5 bar) supply pressure range using fluid viscosities ranging from 220-5 cSt. It was concluded that hydrofluidic servos are practical and that their application in industry will be contingent upon the availability of off-the-shelf components.