Inside photobioreactors, algae are exposed to light/dark fluctuations. In this study the marine green alga Dunaliella tertiolecta was cultivated under short light/dark cycles of 3/3 s, 94/94 ms and 31/156 ms as found in medium and short light-path reactors. The photon flux density (PFD) during the light period was 440-455 mu mol m(-2) s(-1) and, under the 31/156 ms cycle only, 1025 mu mol m(-2) s(-1). The photosynthetic efficiency was determined and expressed as the biomass yield on light energy in gram protein produced per mol of photons absorbed. The yield under the 94/94 ms cycle was higher than the yield under continuous light of 440-455 mu mol m(-2) s(-1). Apparently a light integration effect occurs under the 94/94 ms cycle together with an increase of the photosynthetic efficiency. On the contrary, at the 3/3 s cycle the efficiency decreased. Also under the 31/156 ms cycle, with 1025 mu mol m(-2) s(-1) in the light period, the yield was lower than under continuous light. The results obtained under the 31/156 ms light/dark cycle are discussed with respect to the performance of short light-path flat panel photobioreactors operated at high biomass densities.