Maximal productivity of a 14 mm light-path panel photobioreactor under high irradiance was determined. Under continuous illumination of 2,100 mu mol 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 degrees. 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) (m(2) 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. (C) 2009 Wiley Periodicals, Inc.