It is well-known that the response of photosynthetic microorganisms in photobioreactor (PBR) is greatly influenced by the geometry of the process, and its cultivation parameters. The design of an adapted PBR requires understanding of the coupling between the biological response and the environmental conditions applied. Cells culture under well defined conditions are thus of primary interest. A particular lab-scale PBR has been developed for this purpose. It is based on a torus shape, that enables light to be highly controlled while providing a very efficient mixing, especially along the light gradient in the culture, that it is known to be a key-parameter in PBR running. A complete characterization of hydrodynamic conditions is presented, using computational fluids dynamics (CFD). After validation by comparison with experimental measurements, a parametric study is conducted to characterize important hydrodynamics features with respect to PBR application (light access, circulation velocity, global shear-stress), and then to investigate a possible optimization of the process via modification of the impeller used for culture mixing. The final part of the study is devoted to a detailed investigation of mixing performance of the torus PBR, by numerically predicting dispersion of a passive tracer in various configurations. The high degree of mixing observed shows the great potential of such innovative geometry in the field of photosynthetic microorganisms cultivation, especially for the design of a lab-scale process to conduct experiments under well-controlled conditions (light and flow) for modeling purpose. (c) 2006 Elsevier Ltd. All rights reserved.