The ultrastructure of isolated cell walls of Schizosaccharomyces pombe was studied by electron microscopy after treatment with the following purified enzymes: endo-beta-(1-->3)-glucanase, endo-beta-(-->6)-glucanase, and endo-alpha-(1-->3)-glucanase produced by Bacillus circulans; exo-beta-(1-->3)-glucanase and endo-beta-(1-->3)-glucanase produced by Schizosaccharomyces japonicus var. versatilis. The exo-beta-(1-->3)-glucanase had no detectable effect on the walls, but amorphous wall material was removed by action of the endo-beta-(1-->3)- and endo-beta-(1-->6)-glucanases of B. circulans to reveal a wall component consisting of densely interwoven microfibrils. The fibrils were hydrolyzed by treatment with the Schiz. japonicus endo-beta-(1-->3)-glucanase followed by B. circulans endo-alpha-(1-->3)-glucanase--suggesting that they were composed of -beta-(1-->3)-linked glucan and alpha-(1-->3)-linked glucan. The presence of a fibrillar component in untreated walls was evident after negative staining.