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Carbon fixation in Antarctic nanoflagellates dominated by cryptomonads collected during a summer cruise in 1995 decreased after short‐term exposition (3 h) under both UVA and UVA + UVB radiation compared to white light. The dose applied with artificial lamps was within the range of the natural UV radiation measured at the surface during the cruise. The depletion of C fixation was higher after UVA + UVB than after UVA alone. The inhibition of carbon fixation in the laboratory depended on the time of sample collection and, consequently, on the UV dose received in the natural environment before sampling. Thus, the cells collected in the morning showed 82% of inhibition by UVA + UVB but that collected at noon showed only 72%. The same effect was observed by UVA: 72% of inhibition in the morning samples and 62% at noon. Thus, photoprotection mechanisms seem to be operating during the day protecting …