Two brown algae, Pelvetia canaliculata and Laminaria saccharina, from the higher and lower mediolittoral belts respectively, have been tested for their capacity to overcome high-light stress in water and in air (in both fully hydrated and desiccated states). When exposed to supersaturating light irradiance in water, the two species developed non-photochemical quenching of fluorescence (NPQ) which was correlated with an increase in the de-epoxidation ratio (DR) of the xanthophyll cycle carotenoids (violaxanthin, antheraxanthin and zeaxanthin) and was followed by a slower decrease in oxygen evolution. NPQ reached values of up to 9 in P. canaliculata but only 4·5 in L. saccharina, at DRs of 0·65 and 0·5, respectively. In air, the xanthophyll cycle was also operative but the efficiency of de-epoxidation decreased linearly with the degree of hydration of the thallus. Photoprotection capacities in air also appeared higher in P. canaliculata than in L. saccharina, probably due to the higher molar content of the xanthophyll cycle pool size relative to chlorophyll a (Chl a) in the former (nearly double of that L. saccharina at 19 carotenoid molecules per 100 Chl a), which may be associated with a higher DR at the same level of desiccation. The concomitant higher accumulation of zeaxanthin in P. canaliculata might divert a higher percentage of the incident energy from the reaction centres, as demonstrated by the levels of NPQ, with steady-state fluorescence reduced to below the initial F0 level. Such differences, together with the unequal resistance to desiccation of the operation of the xanthophyll cycle, should be considered as possible factors responsible for the distribution for these two species on the shore.