The objective of this study was to evaluate the effects of hydraulic retention time (HRT) (8 -1 h) on H-2 production from sugarcane juice (5000 mg COD L-1) in mesophilic (30 degrees C, AFBR-30) and thermophilic (55 degrees C, AFBR-55) anaerobic fluidized bed reactors (AFBRs). At HRTs of 8 and 1 h in AFBR-30, the H-2 production rates were 60 and 116 mL H-2 h(-1) L-1, the hydrogen yields were 0.60 and 0.10 mol H-2 mol(-1) hexose, and the highest bacterial diversities were 2.47 and 2.34, respectively. In AFBR-55, the decrease in the HRT from 8 to 1 h increased the hydrogen production rate to 501 mL H-2 h(-1) L-1 at the HRT of 1 h. The maximum hydrogen yield of 1.52 mol H-2 mol(-1) hexose was observed at the HRT of 2 h and was associated with the lowest bacterial diversity (0.92) and highest bacterial dominance (0.52). (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.