Photoinhibition resistance exhibited by the high intertidal red alga Porphyra perforata relative to its subtidal congener Porphyra nereocystis was examined using the protein synthesis inhibitor chloramphenicol to separate the damage and repair components of photoinhibition. Under photoinhibitory conditions, the rates of both damage to and replacement of photoinhibition-sensitive proteins was much higher in P. nereocystis than in P. perforata. Thus, photoinhibition resistance in P. perforata appears to be due to a reduced rate of photoinhibition damage rather than to an accelerated rate of photoinhibition repair. Reduction of photoinhibition damage in P. perforata may be by means of biophysical processes which increase the radiationless decay of excitation to heat in photosystem II. Alternatively, the photoinhibition-sensitive proteins in P. perforata may have slight structural alterations that improve their stability or they may be protected by enzyme systems that quench radicals formed by overexcitation of photosystem II. Reduction of the damage component of photoinhibition is a reasonable way to limit photoinhibition in P. perforata during the severe desiccation and exposure to full sun that occur simultaneously during daily low tides, conditions under which the protein synthesis required for photoinhibition repair could not occur.