Hydrogen production from starch by a co-culture hybrid dark and photofermentation under repeated fed-batch conditions at different organic loading rates (OLR) was studied. Effective cooperation between bacteria in co-culture during initial days was observed at controlled pH 7.0. However, at pH above 6.5 dark fermentation phase was redirected from H-2 formation towards production of formic acid, lactic acid and ethanol (which are not coupled with hydrogen production) with simultaneous lower starch removal efficiency. This resulted in decrease in the hydrogen production rate. The highest H-2 production in co-culture process (3.23 L H-2/L-medium - after 11 days) was achieved at OLR of 1.5 g(starch)/L/day, and it was twofold higher than for dark fermentation process (1.59 L H-2/L-medium). The highest H-2 yield in the co-culture (2.62 mol H-2/mol(hexose)) was obtained at the OLR of 0.375 g(starch)/L/day. Different pH requirements of bacteria were proven to be a key limitation in co-culture system. (C) 2016 Elsevier Ltd. All rights reserved.