Different mechanisms of heat removal (conductive, convective and evaporative) were studied in a SSF process using a packed bed reactor with an inert support (Amberlite IRA -900) previously inoculated. The cylindrical bioreactor was incubated in a temperature controlled chamber (30 degrees C). Average temperature gradients obtained during the culture were 1.3 and 0.42 degrees C cm(-1) in the radial and axial directions, respectively. During maximal metabolic activity of Aspergillus niger the medium temperature rose from 32 to 48 degrees C, and a maximum radial temperature gradient of 3.1 degrees C cm(-1) was achieved. Ar this stage, the unsteady-heat balances indicated that conductive heat transfer was the least efficient mechanism (8.65%) when compared with convective (26.65%) and evaporative (64.7%) mechanisms. Sugar consumption and glycerol and citric acid production were strongly affected by temperature.