Photosynthesis of brown algae in saturating irradiances of red light is stimulated by blue light. Short pulses of blue light lead to transient increases of photosynthetic rates which, in some species, can be up to 4 times the rates in red light. We can distinguish two groups of species that differ with respect to the pattern of their photosynthetic behaviour in red light and the kinetics of blue-light stimulation. The first group contains the orders of the Phaeophyta with little tissue organisation, and the second group those orders that have tissue organisation. In both groups, blue light is thought to activate a carbon acquisition mechanism because (i) the responses to blue light are abolished with increased supply of free CO₂, whereas photosynthetic rates in red light are enhanced, and (ii) the affinity for dissolved inorganic carbon (DIC) is higher in the presence of blue light. The activating mechanism in Group II requires the activity of an extracellular carbonic anhydrase. If DIC is absent from the medium, photosynthesis in species from Group I can still be stimulated by blue light and stimulation in Group II species occurs without the simultaneous enhancement of the rate of uptake of CO₂. We suggest, therefore, that blue light activates the release of CO₂ from an internal store. Possible metabolic pathways for the carbon acquisition systems in both groups of species are presented and a CAM-like metabolism is suggested. In Group II species, the carbon store is thought to be located in tissues separate from the photosynthetic tissues. More recent investigations indicate that the Fucales differ from the rest of the Phaeophyta by additional blue-light activation of the loading of the internal CO₂ store. The mechanism of activation appears to be different under submersed and emersed conditions.