Wastewater treatment processes such as upflow anaerobic sludge bed (UASB)-like reactors are strongly dependent on the aggregation of biomass into macroscopic granules (1-3 mm) which settle well. The effect of the liquid upflow velocity (vUP) on the mean granule size and on the hydrogen, acetate, propionate and glucose specific activities was studied, at various specific loading rates, using sugar wastewater. Reactors operated at 0.9 m h-1 behaved as fixed beds while those run at 2.2,4.4 and 6.6 m h-1 were fluidized. The vUP had a significant effect on granule mean geometric diameter (Dm) and on the specific activities. The Dm increased with the vUP, as a direct function of the hydrodynamic regime, rather than as a result of a specific wash-out of smaller particles at higher vUP. Acidogenic (glucotrophic) activity decreased when vUP augmented. All other post-glucotrophic activities (propionate, acetate, H2) were positively influenced by the vUP. The positive effect of vup increased with the specific loading rate (SLR). Glucose activity gradient within the granule bed was highly and inversely correlated to that of the granule size, while for propionate and acetate activity gradients, the correlation was direct, although less strong. No gradient was observed for H2 activity. These observations are discussed in details with regard to an ordered distribution of the consortium populations within the granule spatial structure. They support the so-called multilayer arrangement of anaerobic granules. These results are encouraging for environmental industries which are promoting UASB-like designs handling anaerobic natural granules under fluidization.