The photosynthetic acclimation of the red macroalgae Gracilaria tenuistipitata at high and low externa1 inorganic carbon concentration (Ci) resembles the characteristics of "shade" and "sun" adapted plants respectively, showing higher photosynthetic rates at low- than at high-Ci. However the photosynthetic efficiency is higher in the low-Ci algae which present a higher pigment content. The influence of inorganic carbon concentration on the levels of photosynthetic pigments and the possible CO, control over photosynthetic apparatus is discussed.