Aspergillus niger germ tubes were exposed for 6 h to 0.15 mg/ml of Congo red, a stain which prevents chitin microfibril assembly. The most evident alterations, detected under ultraviolet light and by transmission and scanning electron microscopy, concerned the hyphal tips which burst or, most frequently, expanded into bulges. In the latter structures, new hyphal tips appeared which, after giving rise to more or less developed hyphae, were themselves converted into new bulges. Therefore, segments derived from isotropic and polarized growth alternated in the organisms exposed to the dye. An interpretation of these abnormalities is advanced based upon the assumption that the maintainance of a regular gradient of wall viscosity in the hyphal extension zone depends primarily on the capability of glycan chains to form crystalline aggregates of increasing complexity.