The composition of the cell walls of synnemal and vegetative hyphae of Ceratocystis ulmi was studied by fractionation and assay of released compounds. Residues after enzymatic hydrolyses were examined by electron microscopy. The synnemal wall was found to have 67% carbohydrate, 4.52% amino sugar, 5.02% protein, 1.6% lipid, and 0.59% ash, which accounted for 78.7% of the cell wall. The vegetative hyphal wall contained 56% carbohydrate, 3.44% amino sugar, 7.92% protein, 4.5% lipid, and 1.45% ash, which totaled 73.3% of the wall weight. Sugars identified were D-glucose, D-mannose, D-galactose, and L-rhamnose. Enzymatic hydrolysis of both wall types by cellulase and laminaranase indicated the presence of beta-1,3 and beta-1,4 linkages of glucose polymers. N-acetylglucosamine was liberated by chitinase. Most of the 16 amino acids detected in each wall type were at least twice as abundant in vegetative hyphal walls as in synnemal hyphal walls. Cellulase and laminaranase treatment of cell walls revealed a fibrillar structure. Chitinase-treated walls did not appear as fibrous, suggesting that the fibrous structure may be mostly chitinous. Synnemal cell walls are covered by electron-dense granules which may correspond to the pigment in the synnemal hyphae.