A novel principle for mixing and aeration in stirred bioreactors, named Variomixing, was developed. Four baffles are rotated intermittently at a rotational speed slower or similar to the speed of a centrally placed axial flow impeller. Rotational speeds of the baffles and impeller of 5-10 and 500-600 rpm, respectively, results in the highly turbulent flow regime characteristic of conventional bioreactors with high mixing and mass transfer capacities. Stagnant zones around crevices and crannies in which wall growth may commence are avoided since the baffles are never completely at rest. Increasing the rotational speed of the baffles (5 s every 5 min), so that it follows the speed of the impeller (500-600 rpm), cancels the effect of the baffles and a deep vortex and high peripheral liquid flow rates at the reactor wall develop. The vortex ensures that also the head-space of the reactor wall is flushed and any deposits removed. The filamentous fungus Aspergillus oryzae has been grown in batch cultures in the Variomixing bioreactor. Compared to conventional laboratory-scale bioreactors, in which more than 30% of all biomass was found attached to walls, less than 2% of the total A. oryzae biomass was found on the walls in the Variomixing bioreactor.