The bag valve comprises an open-ended sleeve of pliant impermeable material, one end of which is removably clamped around the down-stream open end of a conduit for fluids. The other end of the sleeve, the downstream end, can be gathered into a fully or partially closed configuration by a drawstring around the downstream sleeve end. A resilient stopper may be attached inside the downstream end of the conduit, against which the drawstring is biased when fully tightened, in order to minimize leakage from the sleeve. A cleat may be attached to the outside of the conduit for more easily securing the drawstring ends when the valve is partially or totally closed.