Twelve lactating cows divided into three groups of four were fed either equal parts of hay and pelleted concentrate (A), limited hay and pelleted concentrate containing 32% aspen sawdust (B), or limited hay and pelleted concentrate (C), to determine if aspen sawdust could serve as a partial roughage substitute in a high-concentrate ration. There were no significant ration effects on milk production or per cent milk protein. Adjusted means of 2.59, 3.13, and 1.67 for per cent milk fat for Rations A, B, and C were significantly different. Adjusted ruminal acetate: propionate ratios were 3.99, 2.70, and 1.27 for Rations A, B, and C. Time spent ruminating was highest for cows fed Ration B, followed by Rations A and C (5.8, 5.3, and 3.3 hr/day). Dry matter intakes of hay and concentrate on Rations A, B, and C were 7.3 and 7.8, 2.0 and 14.9, and 1.9 and 12.4kg, respectively. Cows receiving Ration B (with aspen) compensated for the lower digestibility of this ration by eating more of it. Thirty per cent dietary aspen sawdust was effective as a partial roughage substitute for hay.