The influence of concentration (mass and volume fraction) and particle morphology on the yielding properties of filamentous broths of Aspergillus niger and Streptomyces levoris was investigated using the rotating vane technique and compared with those of pulp suspensions. Two methods were applied to determine the volume fraction of the cells growing in filamentous form : the measurement of interstitial volume using a high molecular weight dye or dextran, and the measurement of dewatered broth filter cake volume by displacement using a pycnometer. The latter method provided the most reliable results. Cell concentrations ranging from 3 to 20 g dw/L, with corresponding volume fractions between 0.005 and 0.05, were obtained with broths generated in stirred tank and shake flask fermentations. The yield stress values obtained using the vane technique (0.1 less than or equal to tau(y) less than or equal to 37 Pa) correlated with biomass concentration (X, grams dry weight per liter) using the equation tau(y) = aX(b) with "b" ranging between 2 and 3. Increasing the length of organisms significantly increased the yield stress at a given cell concentration; this morphology effect was described using a "morphology factor" (range 0.016 to 0.11 Pa/[g dw/L](2.5)), determined by setting the value of "b" to 2.5. The vane yield strain, which is the critical strain or deformation at the yield point, was independent of cell concentration, but increased with increasing length of organisms. Filamentous fermentation broths with highly branched organisms had a greater yield stress than pulp suspensions.