Cultures of Saprolegnia monoica were treated with Congo red, a dye which prevents glycan microfibril assembly. In the presence of the dye, the fungus developed cell wall thickenings and exhibited aberrant hyphal tips which expanded into spherical swellings. In the thickened cell walls, cellulose was produced as short microfibrils or amorphous material. These morphological anomalies were accompanied by changes in the localization of glucose incorporation into the cell wall which was not limited to the hyphal tips. Congo red uncouples the polymerization and crystallization of cellulose into normal microfibrils and it is suggested that the regular assembly of glucan chains into microfibrils is needed for normal hyphal morphogenesis.