Two approaches are compared for scale-up of solid-state fermentation processes in packed-bed bioreactors, one based on a dynamic heat transfer model, and the other based on a modified Damkohler number. A critical bed height is proposed, being the maximum bed height which can be used without undesirable temperatures being reached in the substrate bed during the fermentation. It depends on the microbial specific growth rate, as well as the superficial velocity and inlet temperature of the air. The critical heights predicted by the two approaches are almost identical suggesting that approaches to scaling-up packed-bed bioreactors based on the modified Damkohler number may be successful. The modified Damkohler number is then used to predict how simple rules of scale-up might perform. Superficial velocities will need to increase with scale, although for practical reasons, it is not possible to increase superficial velocity in direct proportion to height indefinitely. A strategy is proposed to guide experimental programs for scaling-up of packed-bed bioreactors.