A newly developed stirred tank bioreactor for the cultivation of filamentous microorganisms on a milliliter-scale was investigated by means of computational fluid dynamics (CFD). The applied paddle impeller has an asymmetric shape and thus creates a complex liquid vortex in an unbaffled reactor. Size and shape of the formed gas-liquid interface were simulated applying CFD because they cannot be measured experimentally. As expected, higher stirrer speeds correspond to deeper vortices and larger interfacial areas. CFD simulations confirmed stirrer functionality at high liquid viscosities. Volumetric oxygen mass transfer coefficients (k(L)a) were estimated based on the simulated interfaces. Comparison with experimental data showed good agreement. Simulated maximum energy dissipations were in the same order of magnitude as experimental data at varying stirrer speeds. In contrast to these findings, simulated power characteristics showed significant deviations if compared to measured data. One reason may be the challenging turbulence modeling of two-phase flows with strong streamline curvature and the mostly transitional flow regime. Another reason could be the demanding measurement of very small stirrer torques. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.