AbstractA modal has been described for the influence of growth rate and shear stresses in the fermentor upon the morphology of filamentous molds. The main concept of this model is the dynamic equilibrium between growth and breakup of the hyphae. The latter has been approached according to well‐known engineering theories for dispersion of physical systems. Experiments to verify the model with a strain ofPenicillium chrysogenumin batch and continuous culture revealed that the length of the mycelial particles increased with increasing growth rate and decreased with increasing power input her unit mass in the fermentor. Although this was qualitatively in agreement with the presented model, quantitatively the model had to be rejected. Variation of the tensile strength of the hyphae with age and culturing conditions could have been one of the causes of disagreement. Oxygen tension, varied independently from stirrer speed, in the range of 12–300 mm Hg was shown to have no influence upon the morphology. With respect to the question of possibly using high‐energy inputs in industrial mold fermentation in order to decrease hyphal length and suspension viscosity, it was concluded that this is of little practical value. A substantial decrease in hyphal length requires an enormous increase in energy