Abstract: A buoyancy control system is disclosed for maintaining a buoyant vehicle at a controlled depth by jettisoning either a heavy liquid or a light liquid. The control system monitors the depth of the buoyant vehicle and determines the direction and magnitude of vertical velocity. If it exceeds the threshold velocity the system calculates how much fluid to jettison in order to decelerate the vehicle to a target velocity. The positive and negative threshold velocities are programmed for control of the average vertical velocity and for depth maintenance. First and second bladders contain heavy and light liquids, respectively. The bladders are subjected to ambient water pressure and are spring loaded and each is provided with an discharge valve. A pressure sensor monitors the vehicle depth and generates a depth signal. A computer derives a velocity signal from the depth signal and calculates the quantity of liquid which should be jettisoned.