A comparative study of the relative sensitivity to ultraviolet radiation (UVR) among seven marine microalgae from the classes Cyanophyceae ( Synechococcus sp.), Rhodophyceae ( Porphyridium cruentum ), Cryptophyceae ( Rhodomonas salina ), Prymnesiophyceae ( Isochrysis galbana ), Bacillariophyceae ( Chaetoceros sp.), Chlorophyceae ( Dunaliella salina ), and Prasinophyceae ( Tetraselmis suecica ), was carried out. Daily exposure for 4 h over a 4-day period with daily increasing doses of UVR (mainly ultraviolet-B, UV-B) led to a continuous decrease in the effective quantum yield (ϕ PSII ) in Rhodomonas salina , Chaetoceros sp. and Isochrysis galbana . In contrast, Tetraselmis suecica did not show a significant inhibition over the 4-day period of exposure. Synechococcus sp., Dunaliella salina and Porphyridium cruentum were able to restore, at least partially, the values of ϕ PSII during the recovery period (white light only, 20 h), even though a significant decrease in ϕ PSII was also measured in these algae after each exposure. Relative inhibition, defined here as the decrease in ϕ PSII after exposure as compared to the value before exposure and normalised to the latter value (RI = ∆ PSII /ϕ PSII ), was fitted against UV-B irradiance according to an equation of the type y = 1 − (a x) c e −bx , where y is RI, x is the UV-B irradiance (W m −2 ), and a, b and c are constants determined from the fit, using unweighted and weighted irradiances. From the fit, the UV-B irradiance for 50% relative inhibition of ϕ PSII (RI = 0.5, E 50 [W m −2 ]), was calculated for each alga. Rhodomonas salina and Isochrysis galbana exhibited the lowest values of E 50 (0.21 and 0.25, respectively, for unweighted irradiance), being the most sensitive algae to UV-B, while the highest value was calculated for Dunaliella salina (0.47 for unweighted irradiance). The E 50 could not be calculated for Tetraselmis suecica since this alga showed an RI maximum of 0.1, and was therefore considered as a species which was highly tolerant of UV-B. A rise in optical density of each algal cell suspension at 680 and 750 nm, as well as in chlorophyll a content determined from methanolic extracts, was evident over the experimental period in all of the species except for Isochrysis galbana and Rhodomonas salina . In these two algae, in contrast to the other species, there was a notable increase in the carotenoid/chlorophyll a ratio, although this had negligible effects for the protection against deleterious UV-B. The phycobilin/chlorophyll a ratio showed a gradual decrease with time in the three phycobilin-containing algae, but it was particularly evident in R. salina after the third day of exposure.