Bubble size distributions were measured at different locations in a 2.8 m(3) portable industrial scale sub-aeration flotation cell, treating zinc cleaner feed in the Hellyer Concentrator in Tasmania, Australia The cell was fitted in rum with four different impeller-stator systems, and operated over a range of airflow rates and impeller speeds. The mean bubble size was found to increase with increase in airflow rate at different locations in the cell, for all four impellers, and to decrease with increase in impeller speed. The mean bubble size was largest close to the impeller shaft and smallest at the impeller discharge point for all the impellers.＇ The shape of the bubble size distribution also changed with location in the cell. The ＇＇global mean＇＇ bubble size calculated by simple arithmetic average of the values at sir locations in the cell coincided remarkably well with the mean bubble size measured halfway between the impeller shaft and the side of the cell, at the top of the pulp. In general, the impellers produced ＇＇global mean＇＇ values of 1.0 mm or less at the manufacturer＇s recommended impeller speed.