A back pressure regulating valve for ultra high pressures. The valve is designed for use in throttling ultra high pressure pumps, such as those used in fluid jetting systems. The valve has an elongated tapered poppet which fits in an elongated tapered seat such that an elongated, annular tapered flow path is defined therebetween. The flow path preferably has a length greater than a diameter of the poppet, such as approximately ten times the maximum diameter. Also in the preferred embodiment, a minimum annular tapered flow path is defined between the seat and poppet even when the poppet is in an unactuated position. Biasing is provided by a gas filled pressure chamber acting on an actuator piston. The piston at least partially defines a damping chamber, and an orifice is provided between the damping chamber and the pressure chamber so that movement of the piston is damped as it is moved in a direction tending to reduce the volume of the pressure chamber.