Gas holdup is one of the most important hydrodynamic parameters needed for reliable design, performance estimation, and scale-up of stirred tank reactors (STRs). In the present work, local gas holdup is measured in an acrylic stirred tank reactor equipped with a Rushton impeller using X-ray computed tomography (CT) for many different operating conditions. Power consumption for different operating conditions is determined to identify various STR flow regimes. The gas holdup results obtained by X-ray CT imaging are presented as: (i) profiles along all 3 axes, (ii) plots of local gas holdup along the x-axis, (iii) average gas holdup for z-slice, and (iv) overall gas holdup for the imaging region. The high resolution of the X-ray CT system allows for the visualization of minor details such as recirculation zones behind the baffles. The results show that there are dramatic differences in gas dispersion depending on the flow regime. Completely dispersed conditions have a relatively constant holdup profile while flooded conditions have a parabolic shape with an increase in gas holdup towards the center of the tank. The CT slices show that there is very little visual difference between scans taken in the same operating regime, even though there are differences in impeller speed and gas flow rate. Average z-slice holdup values increase with increasing height from the impeller for the flooded condition, while the opposite occurs for the loaded and completely dispersed conditions. Local gas holdup conditions are sensitive to tank design, which are shown by differences in the x- and y-slices. Overall holdup values for the image region are determined and shown to increase as the impeller speed increases while holding Qg constant.