Approximately 4 billion gallons of fuel ethanol was produced in 2005 in the United States, mostly from corn grain, and production is expected to continue to increase in the future. The major coproduct is distiller's dry grain and solubles (DDGS), which is sold as an animal feed. The increase in DDGS supply is expected to drive its value down, and thus, further value addition is necessary. One approach is to convert the cellulose in DDGS into sugars for increased ethanol production, leaving a residue higher in protein content. In this work, the effect of ammonia fiber expansion (AFEX) pretreatment on the enzymatic hydrolysis of both wet and dry DDGS was studied. Hydrolysis of AFEX treated samples gave 190 g glucose/kg dry biomass, or virtually complete conversion of cellulose after 72 h. Optimal AFEX conditions for dry and wet DDGS were 70 degrees C and 0.8 kg anhydrous NH3/kg dry biomass and 80 degrees C and 0.6 kg NH3/kg dry biomass, respectively. Xylose yields were negligible.