A sunlight simulator was used to investigate the effects of short term irradiation on photosynthetic oxygen production and chlorophyll fluorescence in the unicellular algae Dunaliella salina and Ochromonas danica. Oxygen evolution was measured with a Clark-type electrode and initial (Fo) and maximal (Fro) fluorescence were determined by means of a pulse amplitude-modulated (PAM) fluorometer. Using different long-pass filters, a UV exclusion study was performed comparing the effects of irradiation with full sun simulator light, with sun fimulator light without UV-B and short wavelength UV-A radiation and without any UV component. In D. salina, exposure to full sun simulator light caused a marked drop in photochemical efficiency indicated by a decline in the initial slope of the irradiance-response curve tbr photosynthetic oxygen production and by a decrease in the ratio Fv/Fm, whereas only a slight inhibition of photosynthetic activity was 3bserved irradiating with PAR and UV-A radiation or with PAR alone. In the case of O. danica, a strong inhibition was observed under all ~rradiation conditions. The results are consistent with preliminary data obtained with outdoor experiments.