Three studies were conducted to determine the effects of replacing rolled barley grain with either 20% or 40% diet dry matter (DM) of wheat dried distillers' grains with solubles (WDDGS) in Merino wether diets. In the in vitro study, gas and ammonia production were not affected by dietary treatment. The addition of WDDGS in the diet increased total volatile fatty acids, in vitro DM digestibility (IVDMD) and slightly lowered ruminal culture pH. In the in situ study, the WDDGS diets demonstrated faster rate of neutral detergent fibre degradation (k, h(-1)) compared with control diet. In the in vivo study, dry matter intake was about 14% higher for sheep fed 40% WDDGS compared to control or 20% WDDGS. The inclusion of WDDGS in the rations improved greasy and clean wool production, and staple length. This study showed up to 40% WDDGS could replace rolled barley grain in Merino sheep rations to improve wool production.