Ultraviolet-screening capacity of macrothalli from marine chlorophytes was analyzed using an in vivo technique based on chl fluorescence. The method, originally introduced to assess epidermal UV transmittance in leaves from higher plants, is extended to macroalgae. Validation of the method was obtained by measuring unprotected samples (i.e., isolated chloroplasts from six algal species). It is shown that in a total of 71 investigated green macroalgae, including cultured and field-collected material from six systematic orders, only 40% or 60% displayed significant screening of ultraviolet-A (UVA) or ultraviolet-B (UVB) radiation, respectively. Generally, the extent of screening was low in most of these species. Data analysis resulted in a clear phylogenetic pattern with minor influence of climatic origin of a given species. For some species, comparison between field-collected and culture-grown samples was possible. Only in 11 of 25 species field collected algae had appreciably higher screening than those grown in the absence of UVB radiation. For the first time, very efficient UVA and UVB screening is demonstrated for the order of the Cladophorales. Their UVB-screening potential varied between 40% and 85% of incoming UVB radiation. However, the nature and localization of the detected UV-absorbing compounds are still unknown. Long-term UV-exposure experiments pointed to a negative correlation of UVB-screening capacity and UV-induced inhibition of photosynthetic efficiency. Thus, species with pronounced screening were more UV resistant than species with lower screening.