Biohydrogen production from sugar beet molasses was investigated by using agar immobilized R. capsulatus YO3. A panel photobioreactor (1.4 L) was employed for a long-term hydrogen production in both indoor and outdoor conditions. The impact of several initial molasses concentrations on hydrogen production, yield and productivity were assessed. Indoor studies revealed that initial sucrose concentration in molasses should be kept below 20 mM to prevent inhibition of hydrogen production. The highest hydrogen productivity of 0.64 +/- 0.06 mmol H-2 L-1 h(-1) and yield of 12.2 +/- 1.5 mol H-2/mol sucrose were obtained in indoors throughout 20 days of operation. For outdoors, hydrogen production continued for 40 days including consecutive 10 rounds under natural outdoor conditions. In outdoor conditions, the maximum hydrogen productivity and yield were 0.79 +/- 0.04 mmol H-2 L-1 h(-1) and 5.2 +/- 0.4 mol H-2/mol sucrose respectively. These results indicate that the proposed system is promising for biohydrogen production from molasses at large-scale natural conditions. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.