SUMMARY The metabolic stability of carbohydrates in walls of hyphae of Aspergillus clavatus was investigated through measurements of mycelial isotope distribu- tion from (U14C)glucose and the ,H/14C ratios of re-isolated monosaccharides, when the organism was grown on labelled glucose as sole carbon source. The results suggest that glucose and glucosamine became metabolically inert once they had been incorporated into the wall polymers, and that the bio- synthesis of glucosamine and its incorporation into the cell wall as N-acetyl- glucosamine were direct processes not subject to metabolic randomization at the precursor level.