The effect of biomass concentration and mycelial morphology on fungal fermentation broth rheological properties has been investigated. in previous work it had been shown that commonly used rheological parameters, such as the power law consistency and flow behavior indices, could be correlated successfully with the broth biomass concentration and clump morphological parameters of roughness and compactness. More recent work on a broader range of data showed a correlation between roughness and compactness; consequently, it was not correct to use both of these morphological variables simultaneously in rheological parameter correlations. Furthermore, earlier correlations were only made using clump morphological parameters, as clumps were found to be around 90% of the biomass in batch fermentations. In the present work it was found that the percentage of clumps fell to around 30% to 40% of a sample during the later stages of fed-batch fermentations. No clear relationship between the flow behavior index and biomass concentration was found, at least for those phases of the fermentation in which the viscosities were high enough for the rheology to be characterized by a disk turbine rheometer. The mean value of the flow behavior index was found to be 0.35 +/- 0.1 (standard deviation) throughout both batch and fed-batch fermentations, although some significant deviations from this value were observed early and very late in the fermentations. Correlations for the consistency index, measured using a disk turbine rheometer, were based on the biomass concentration and the mycelial size (represented by the mean projected area or the mean maximum dimension of all the mycelia). These correlations were reasonably successful for both fed-batch and batch fermentations. The correlation using the mean maximum dimension was preferred to that using the mean projected area, as the former is independent of magnification. The proposed correlation is: K = C-m(2) x (5 x 10(-5) D - 10(-3)) where K is the consistency index (Pa . s(n)), C-m is the biomass concentration as dry cell weight (g L-1), and D is the mean maximum dimension (mu m). It should be noted that small changes in the exponent on the biomass concentration (alpha) may dramatically affect any predictions. Consequently, caution in the use of this correlation (and that based on mean projected area) is advocated, although its accuracy may be suitable for operational or design purposes. (C) 2000 John Wiley & Sons, Inc.