Production of conidia of the biocontrol fungus Coniothyrium minitans by solid-state cultivation in a packed-bed reactor on an industrial scale is feasible. Spore yield and oxygen consumption rate of C. minitans during cultivation on oats and three inert solids (hemp, perlite, and bagasse) saturated with a liquid medium were determined in laboratory-scale experiments. The sensitivity of the fungus to reduced a(w), and the water desorption isotherms of the four solid materials were also determined. C. minitans is very sensitive to reduced a(w): 50% inhibition of respiration was found at a(w) 0.95, spore formation was completely inhibited at a(w) 0.97. A simplified mathematical model taking into account convective and evaporative cooling was used to simulate temperature and moisture gradients in the bed during cultivation, Adequate temperature control can be achieved with acceptable air flow rates for all four solid matrices. Moisture control is the limiting factor for cultivation in a packed bed. Oats cannot be used due to the shrinkage and a(w) reduction caused by evaporative cooling. Of the three inert supports tested, hemp provides the best spore yield and control of water activity, due to its high water uptake capacity. A spore yield of 9 x 10(14) conidia per m(3) packed bed can be achieved in 18 days, using hemp impregnated with a solution containing 100 g dm(-3) glucose and 20 g dm(-3) potato extract. Sufficient water is predicted to be available after 18 days, to allow a higher initial nutrient concentration, which may lead to higher spore yields.