Dynamic viscosity measurements and rotating vane tests on filamentous broths of Aspergillus niger and Streptomyces levoris showed that filamentous suspensions are often highly viscoelastic and possess a yield stress. The storage modulus of filamentous fermentation broths of A. niger was greater than the loss (viscous) modulus at very small deformations, but the storage modulus decreased more rapidly than the loss modulus as the strain increased. The "static" vane yield stress was compared to the "dynamic" yield stress measured by extrapolating the shear stress-shear rate data using the Casson and Herschel-Bulkley rheological models. For S. levoris broths, there was a close agreement between the vane yield stress and those obtained by extrapolation; however, for A. niger broths the vane yield stress was much greater than those determined by conventional methods. Larger particles, more heterogeneous characteristics, and therefore highly viscoelastic properties of the A. niger broths were mainly responsible for the higher "static" yield stress. The vane technique measures a combination of the viscous and elastic properties and provides a realistic representation of the "true" yield stress at zero shear rate that may be significantly higher than the yield stress obtained by extrapolating shear stress-shear rate data to zero shear rate.