Data are presented which show that the photosynthetic and respiratory characteristics of the giant-celled alga Chaetomorpha darwinii are those of a shade plant (denned physiologically in terms of rates of photosynthesis at light and inorganic carbon source saturation, and the irradiance required to saturate photosynthesis and to compensate respiration by photosynthesis). This is in agreement with previous suggestions that giant-celled algae have the characteristics of ‘shade plants’, while small-celled macroalgae can be either ‘sun plants’ or ‘shade plants’. A comparison of giant-celled algae with small-celled macro-algae in terms of ecological strategies has been made, using previously published data on Enteromorpha intestinalis, Chaetomorpha linum (‘fugitive’small-celled), Chaetomorpha darwinii, Hydrodictyon africanum (‘stressed’ giant-celled), Cham corallina (‘canopy-dominant’/‘stressed’ giant celled) and Laminaria digitata (‘canopy-dominant’ small celled), as well as new data on net and mass spectrometric measurements of O2 exchange in L. digitata. It is suggested that the ability of giant-celled algae to compete as ‘canopy dominants’ may be related to cytoplasmic streaming in the ‘absence of many (or any) plasmodesmatal barriers to diffusion providing an efficient means of long-distance transport in a differentiated plant, as an alternative to phloem-type translocation in small-celled ‘canopy dominants’. The significance of the giant-celled habit in ‘stressed’ macroalgae is not clear, although the absence of giant-celled algae from the ‘fugitive’ category may be related to their limited capacity for rapid growth relative to small-celled algae.