An apparatus for mixing gas and liquid is disclosed. The apparatus may have a plurality of blades rotated about a common shaft. The blades may have a forward portion, which cuts through a liquid, and a trailing portion, which forms the leading part of a gas cavity. The gas cavity is in communication with a gas inlet and the bulk fluid. As the blades pass through a liquid, a reduced pressure zone is formed immediately behind the blade. The reduced pressure zone allows air to be aspirated from the gas inlet into the gas cavity adjacent to the blade. Gas in the gas cavity is sheared into discrete bubbles and transferred to the liquid by the action of the trailing blade wake currents.