Technical Abstract: Neutral detergent fiber (NDF) is considered the single best laboratory predictor of voluntary intake by ruminant livestock, creating interest in using NDF as a selection criterion in forage breeding programs. Because genetic reductions in NDF lead to increases in dry matter digestibility, but not to changes in digestibility of the NDF fraction, we postulated that low-NDF plants do not have altered composition of their cell walls. We tested this hypothesis using clones of smooth bromegrass (Bromus inermis Leyss.) with divergent NDF concentration. High-NDF and low-NDF plants did not differ in cell-wall concentration or in the concentration of any cell-wall component (fucose, arabinose, rhamnose, galactose, glucose, xylose, mannose, total uronosyls, and lignin). Instead, low-NDF plants had a cell wall that was more susceptible to solubilization in neutral detergent solution, suggesting that their cell walls were less well developed compared to high-NDF plants. Neutral detergent fiber should not be used as a substitute for cell-wall concentration in forage plants.