The effect of agitation, due to a Rushton turbine, on the bubble size and bubble size distribution has been studied in an aerated vessel. By using a noncoalescing system, it has been possible to study the bubble breakup process in isolation. Bubbles of two different sizes, with d32 of 300 mum and 2.5 mm, are generated using porous spargers. With no agitation, the bubble populations are found to exhibit a log-normal distribution. The 300-mum bubbles are too small to undergo further breakage for the range of impeller speeds studied, and the size distribution is preserved. The larger bubbles are broken by agitation and the d32 decreases. Also with increasing agitation, the size distribution changes from unimodal to bimodal, and again to unimodal, as the bubble population moves progressively down the size scale. A model proposed, based on the energy dissipated in the impeller zone, predicts the continually changing pattern of the bubble size distributions satisfactorily.