Large scale culture of marine dinoflagellate microalgae has proved difficult owing to their extreme sensitivity to hydrodynamic forces in photobioreactors. This work discusses the scale up of Protoceratium reticulatum dinoflagellate culture from a successful 2 L stirred-tank photobioreactor operation to a 15 L stirred photobioreactor. Both bioreactors were equipped with internal spinfilters for cell retention. A semicontinuous perfusion culture in the 15 L photobioreactor proved to be more productive than fed-batch perfusion culture. Under the best operational conditions, the average cell productivity in the 15 L photobioreactor was 5228 cell mL(-1) day(-1), or nearly 3.7-fold greater than the best attainable value in static flask cultures, but similar to the results obtained in the 2 L stirred tank. At 9.16 mu g L-1 day(-1) the average volumetric productivity of yessotoxin in the scaled up operation in the semicontinuous perfusion mode was comparable to the results obtained in the small bioreactor. P. reticulatum has a cell damage threshold shear rate of as low as 0.1 s(-1). Notwithstanding this extreme shear sensitivity, the results suggest a good potential for mass scale culture of this dinoflagellate in suitably designed photobioreactors. (c) 2011 Elsevier Ltd. All rights reserved.