A two-dimensional heat transfer model was validated against two experimental studies from the literature which describe the growth of Aspergillus niger during solid-state fermentation in packed bed bioreactors. With the same set of model parameters, the two-dimensional model was able to describe both radial temperature gradients, which dominated in one of the studies, and axial temperature gradients, which dominated in the other study. The sensitivity of the model predictions to the characteristics of the substrate and the microbe were explored. The temperatures reached in the column are most sensitive to parameters which affect the peak heat load, including the substrate packing density, the maximum specific growth rate, and the maximum biomass concentration. Even though the bed is assumed to be aerated with saturated air, the increase in temperature with bed height increases the water-carrying capacity of the air and therefore enables evaporation to contribute significantly to cooling. The model suggests that evaporation can remove as much as 78% of the heat from the bed during times of peak heat generation. Our model provides a tool which can guide the design and operation of packed bed bioreactors. However, further improvements are necessary to do this effectively, the most important of which is the incorporation of a water balance. (C) 1998 John Wiley & Sons, Inc.