Microporous, hollow fiber membranes have potential for use as a lake hypolimnetic aerator because oxygen is transferred without bubble formation. Thus, the large capital costs of traditional hypolimnetic aerators can be avoided. Two hollow fiber aerator designs are considered for application to hypolimnion aeration. The first was a passive design that required no external energy input other than a pressurized oxygen supply. This approach was dismissed, however, because the low velocities which exist in lake hypolimnia result in oxygen transfer rates that are too small to provide cost-effective aeration. An alternative design, in which membrane modules were rotated through the water by a mechanical motor, was also examined. Experiments were conducted to characterize fiber performance for this design as a function of relative water velocity, and examine the effects of fouling on membrane performance. The experimental results were used to assess capital and operating and maintenance (O&M) costs of a rotating membrane aerator. The estimated costs compare favorably with the known costs of conventional lake aerators, indicating that this is a potentially viable new technology.