The effects of salinity on the primary processes of photosynthesis were studied in the red alga Porphyra perforata. The results show that there are at least three sites in the photosynthetic apparatus of this alga that were affected by increased salinity. The first site, photoactivation and dark-inactivation of electron flow on the reducing side of photosystem I, was completely inhibited at high salinity. The second site, electron flow on the oxidizing side (water side) of photosystem II, was inhibited as was the re-oxidation of Q in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. The third site affected by high salinity was the transfer of light energy probably from pigment system II to I. High salinity reduced the amount of light energy that reached the reaction centers of photosystem II.These effects are discussed in relation to the mechanisms available to this plant to avoid photoinhibition when it is exposed to stresses such as high light and high salinity which are conditions that are commonly found in the intertidal habitat.