Continuous production of hydrogen from sugary wastewater by anaerobic microflora in chemostat culture was examined as a function of hydraulic retention time (HRT) in the reactor. The measured volumes of the evolved gas at each HRT were almost constant (Avg. 3590 mI/l-feed) and the composition of the gas was approximately 64% hydrogen, 36% carbon dioxide, and less than 0.13% methane. Steady states on evolution of gas were observed for 190 d at HRTs from 0.5 to 3 d giving hydrogen production rates from 198 to 34 mmol/l/d. Significant amounts of acetate and butyrate were formed as by-products. A maximum production yield of hydrogen of 14 mmol/g carbohydrate removed was obtained at an HRT of 0.5 d. The maximum removal efficiency of carbohydrates was approximately 97% at an HRT of 3 d. The patterns of fermentation by anaerobic microflora changed with HRT, i.e., acid formation decreased with decreasing HRT.