Radial solids concentration profiles were determined with a fiber-optic probe on eight axial levels in a 76-mm-ID, 10-m riser, at solids flux (solids circulation rate) up to 550 kg/m(2.)s and superficial gas velocity up to 10 m/s. Radial concentration profiles at high solids fluxes of over 300 kg/m(2.)s are less uniform than lower fluxes of less than 200 kg/m(2.)s. Under all operating conditions, the flow development in the riser center is nearly instant, with the solids concentration remaining low at the riser center throughout the riser. In the wall region, increasing solids flux significantly slows down the flow development, with the solids concentration near the wall decreasing all the way toward the riser top at high fluxes. In the high-flux circulating fluidized bed (HFCFB), a middle section with intermediate solids holdups of about 7 to 20% was between the bottom dense section and top dilute section, with its length increasing as the solids circulation rate increases and as the gas velocity decreases. Flow conditions in this section resemble those of the dense suspension upflow tinder high-density operating conditions by Grace et al. (1999). When its length extends to the riser top, a high-density circulating fluidized bed for which an HFCFB is a necessary but not a sufficient condition, forms.