Pigment composition, fluorescence parameters, and oxygen evolution of the deep water Laminaria abyssalis Oliveira and of the shallow water L. digitata Lamoroux were determined in response to high irradiances. This was performed in the presence and absence of an inhibitor of violaxanthin de-epoxidase (dithiothreitol) or an inhibitor of the chloroplast-encoded protein synthesis (chloramphenicol). Photochemical quenching in L. digitata was almost 3-fold that seen in L. abyssalis, whereas both nonphotochemical quenching and PSII photochemical yield were doubled. Laminaria digitata possessed a xanthophyll-cycle pool nearly double that of L. abyssalis. After photoinhibitory treatment, L. digitata displayed substantial violaxanthin de-epoxidation, whereas in L. abyssalis de-epoxidation only took place in limited amounts. Both species were able to fully recover their epoxidation status after transfer back to dim light. Overnight incubation with dithiothreitol fully blocked de-epoxidation in both species, and both displayed similar fluorescence properties. Chloramphenicol caused no change in their fluorescence parameters. With high light treatment, L. abyssalis was completely and irreversibly inhibited both in the presence and absence of inhibitors, whereas L. digitata showed 60% inhibition of its photosynthetic activity and full recovery in the absence of inhibitors. In the presence of dithiothreitol, L. digitata did not recover to the preillumination conditions and chloramphenicol delayed the recovery of the oxygen evolution activity. We suggest that the xanthophyll cycle is the main mechanism of photoprotection of these Laminaria species and that the higher susceptibility of L. abyssalis to photoinhibition may be due to its limited de-epoxidation capacity and reduced xanthophyll-cycle pool size.