An aerator device and method of aerating which comprises a hollow-bladed axial impeller means to effect supercavitation below the surface of the liquid and to produce a downward, swirling flow of gas and liquid; means for admitting a flow of gas to and through a first venturi and out through the blades of the propeller; wall means surrounding said blade means for cooperatively forming therewith an effective second venturi adjacent the blades; and means for preventing gas having passed the blades from reentering the blade area before being dissolved in the liquid.