The formation of large standing bubbles in the sparged downcomers of U-tube fermenters is undesirable. The behaviour of such bubbles has been investigated experimentally in a verticle pipe of diameter 5.7 cm. The rate of entrainment of air from the bubble base by the liquid falling around it is found to determine the length of the bubble. If air is injected. through holes placed around a diameter of the downcomer wall, as opposed to through a sparger placed horizontally across a diameter or vertically along the downcomer centre line, the bubble length is reduced. A decrease in the surface tension of the liquid being used also decreases the length of the bubble. An increase in viscosity increases the length of a bubble formed. Power law fluids also increase the length of bubble produced