The focus of this study is the movement of the fluid contained in a rectangular bioreactor agitated by line-source bubble plumes. The research included visualization of fluid motion by white polystyrene beads and supported by intensive video recording. Water velocity at the surface and at the bottom, as well as the velocity along the depth, have been measured in two aerated rectangular vessels (36 x 10(-3) m(3) and 2 m(3) in volume) using tap water, as a function of the characteristic velocity (qg)0.33. It was confirmed that the best location of the perforated tube was in the middle of tank and the minimum air flow rate to maintain particle suspension was reached when the aerated surface was a rectangular surface in which the length is twice the width. In addition, the relationship between the fluid velocity, air flow rate, height of the liquid and flow patterns were discussed. Finally, a correlation for the velocity in the liquid was developed and found to be adequate in the scale-up of aerated tanks agitated by an air curtain.