Photosynthesis versus irradiance curves, and their photosynthetic parameters, as well as photosynthetic pigment contents and dry weight (DW):fresh weight (FW) ratios, were studied in gametophytes, young sporophytes and old sporophytes of the brown alga Ecklonia cava Kjellman. Gametophytes had the highest pigment concentrations [Chlorophyll (Chl) a , Chl c and carotenoids] when expressed on a FW basis, followed by young and old sporophytes. However, per area unit, old sporophytes showed higher pigment contents than young sporophytes. Dry weight:fresh weight ratios were twice as high in blades of old sporophytes as in those of young sporophytes, and old sporophytes had fifteen times higher DW per surface unit than young sporophytes. The maximum photosynthetic rate was significantly higher in gametophytes than in sporophytes when expressed on a FW basis, but when this parameter was expressed on Chl a and area basis, old sporophytes had the highest values. Dark respiration rates on a FW basis were significantly higher in gametophytes and young sporophytes than in old sporophytes. Photosynthetic efficiency (α) based on FW basis was higher in the early developmental stages, i.e., gametophytes and young sporophytes. In the sporophytic stage, when α was expressed on area basis, old sporophytes showed higher values than young sporophytes. Compensation light level (I c ) and light saturation parameter (I k ) were significantly lower in gametophytes and young sporophytes than in old sporophytes. These differences in photosynthetic performance among developmental stages may be related to thallus structure and light conditions at growth sites. The results of this study suggest that gametophytes and young sporophytes exhibit the characteristics of ‘shade-adapted’ plants, i.e., high pigment contents, high α, and low I c and I k , which serve to optimise photosynthetic performance under the canopy of adult sporophytes.