Local voidages were determined for conditions corresponding to dense suspension upflow and fast fluidization in a riser of diameter 76.1 mm and height 6.1 m using a reflective-type optical fiber probe at superficial air velocities between 4 and 8 m/s and solids circulation fluxes up to 425 kp/m(2) s. High-density flow of cross-sectional mean volumetric solids concentration of about 0.2 +/- 0.05 was achieved in the riser with fluid catalytic cracking (FCC) particles of mean diameter 70 mu m and density 1600 kg/m(3). Local time-mean voidages were nearly as low as epsilon(mf) at the wall and as high as 0.9 at the axis. As in our other recent work, solids refluxing near the wall of dilute CFB risers no longer existed for high-density conditions. Statistical analysis of local voidage fluctuations was used to characterize the local flow dynamics. The core behaves as a relatively uniform dilute flow, interspersed with infrequent particle structures. The number and density of these structures increases with radius, Maximum heterogeneity for high-density conditions occurs at some distance from the wall, unlike dilute risers where the flow is least uniform at the wall. intermittency index vs. local voidage plots are "bell-shaped". with a maximum where the local time-mean voidage is approximately 0.75.