A discussion is presented suggesting that hyphal breakage in mechanically stirred fermenters is likely to occur by direct tensile stresses acting on the opposite ends of the hypha of filamentous microorganisms. These stresses originate from the dynamic pressure fluctuations of eddies with scales in the inertial convection subrange of the turbulent energy spectrum. The maximum strain energy theory of failure of an elastic material under this mode of stress is used to set up a relationship between the stable length of hyphae and some of the physical parameters affecting it, including the mean energy dissipation rate in the bioreactor. Experimental data are reported for the rate of breakage of hyphae obtained under different operating conditions during the fermentation of Penicillium chrysogenum in a 7 and 150 1 mechanically stirred bioreactor. Data on the stable size of hyphae agree well with the model based on the maximum energy criterion. The experimental data further suggest that hyphal breakage is approximately a first-order kinetic process with a rate constant which appears to be moderately dependent on the product of the mean energy dissipation rate and the reciprocal of the impeller circulation time.