Abstract The photosynthetic response to light of the marine phanerogam Posidonia oceanica (L.) Delile and its epiphytes from the Bay of Port-Cros (France) was determined monthly from March 1981 to July 1982. A 14C technique was developed in situ for entire plants of P. oceanica. Their epiphytes were then removed in the laboratory after incubation. The productivity increased with light to an asymptotic value at which the system was light saturated. During summer photoinhibition was observed. In spite of a high variability, epiphytic productivity was twice that of P. oceanica. The productivity-irradiance relationship showed both circadian and seasonal variations. During the year, productivity-irradiance curves of P. oceanica and its epiphytes followed the sun-shade model for photosynthetic adaptation. The photosynthetic efficiency was highest in winter and decreased towards summer. During the day photosynthetic efficiency was highest in the morning and decreased towards sunset. Factors which may regulate these variations in efficiency are discussed.