Maximal productivity of a 14 mm light-path panel photobioreactor under high irradiance was determined. Under continuous illumination of 2,100 µmol photons m−2 s−1 with red light emitting diodes (LEDs) the effect of dilution rate on photobioreactor productivity was studied. The light intensity used in this work is similar to the maximal irradiance on a horizontal surface at latitudes lower than 37°. Chlorella sorokiniana, a fast-growing green microalga, was used as a reference strain in this study. The dilution rate was varied from 0.06 to 0.26 h−1. The maximal productivity was reached at a dilution rate of 0.24 h−1, with a value of 7.7 g dw m−2 h−1 (m2 of illuminated photobioreactor surface) and a volumetric productivity of 0.5 g dw L−1 h−1. At this dilution rate the biomass concentration inside the reactor was 2.1 g L−1 and the photosynthetic efficiency was 1.0 g dw mol photons. This biomass yield on light energy is high but still lower than the theoretical maximal yield of 1.8 g mol photons−1 which must be related to photosaturation and thermal dissipation of absorbed light energy. Biotechnol. Bioeng. 2009; 104: 352–359 © 2009 Wiley Periodicals, Inc.