Axial distribution of gas holdup was measured by static pressure difference and a void sensor in a square bubble column. There were three regions -the entrance region, the bulk region and the foam layer- in the bubble column. epsilon(e), gas holdup in the entrance region, was almost zero and epsilon(b), gas holdup in the bulk region, was axially constant. Gas holdup in the foam layer increased with height above the gas spargers, The clear liquid height and kind of gas sparger had no influence on the form of the axial distribution of static pressure and gas holdup. The thickness of the foam layer and the entrance region was correlated. The effect of the foam layer on the gas holdup was large and was correlated.