Experiments on microporous, polymeric, hollow fiber membranes are being conducted for application to in-stream aeration. The hollow fibers are scaled on one end and connected to an oxygen supply on the other, so that they may be filled with pressurized oxygen gas. Oxygen transfer into the water occurs through the pores, which are less than 0.1 μ m in diameter and will not allow bubbling to occur except at high pressures. The hollow fiber membranes may thus be visualized as long cylindrical bubbles that do not rise and approach 100 % transfer efficiency. The fibers are fixed at the oxygen supply end and are free at the sealed end so that they will move and clean themselves similar to grass growing on a stream bed.