Metarhizium anisopliae was grown by solid-stare fermentation using a medium based on a mixture of rice bran and rice husk (1:1 on a dry weight basis) where the initial water content was fixed at a value of 470/0. The experiments were performed in glass column bioreactors of three different sizes. Moist saturated air was continuously passed through the bottom of each column at a flow rate of 0.34 1 h(-1) g(-1) initial dry matter, and cultivation was performed for two weeks at 27 +/- 1 degrees C. The effect of medium packing density was studied in the columns of small size. For initial apparent densities of 0.270 and 0.357 g ml(-1) no significant differences were observed either in total biomass production or spore yields; however, when the initial apparent density was increased up to a value of 0.496 g ml(-1), both growth and sporulation were strongly affected. Thermal gradients in the medium and larger columns packed at an apparent density of 0.357 g ml(-1) were observed both in the radial and axial directions. Heat build-up effects were analyzed in the larger column. In this case, there were significant differences between different segments of the column both in the water content of the fermented matter and the sporulation pattern.