The effect of inorganic (IC) depletion on fluorescence quenching was studied under laboratory conditions using the chlorococcal alga Scenedesmus quadricauda strain Greifswald/15. The absence of IC caused a decrease in photochemical quenching (^p), fluorescence yield and the photosystem 2 photochemical yield (dF/F^), and an increase of non-photochemical quenching {q^). High extemal pH (about 11), which accompanies the IC-depletion, did not háve any remarkable effect on the algae. Fluorescence parameters were reversed by a resupply of CO2. The IC-defícient algae when exposed to high irradiance appeared to be less sensitive to the photoinhibition than the algae supplied with CO2. Increased thermal dissipation of the excitation energy (non-photochemical quenching) in the absence of IC is one of the probable protective mechanisms against photoinhibitory damage.