Type A choking velocities, which set the boundary between fast fluidization and cocurrent upward transport, have been determined experimentally in a 152 x 152 mm riser based on differential pressure fluctuation and local voidage fluctuation measurements. The overall volumetric solids concentration at Type A choking velocities, i.e. the saturation concentration, rarely exceeds 0.01 for large Group B and Group D particles, but can be as high as 0.03 for tine Group A particles. A critical velocity U-se, which marks the onset of significant particle entrainment from the riser, delineates the boundary between turbulent fluidization and high velocity fast fluidization. The critical velocity is much higher than the particle terminal settling velocity for Group A particles, while the two velocities are almost equal for Group D particles. Recent information is incorporated on an extended flow regime map.