The internal salt content and distribution in photosynthetic tissues as well as the effect of NaCl on photosynthetic carbon fixation enzymes was investigated in two seagrass species from the Red Sea.Concentrations of both Na+ and Cl- were lower in the chloroplast-rich epidermis than in underlying cell layers in Halophila stipulacea. In Halodule uninervis, the concentration of Na+ was lower in the epidermis than in the underlying cells, while K+ was evenly distributed between cell layers. The epidermal concentrations of Na+ were estimated to be 0.17 and 0.10 M for Halophila stipulacea and Halodule uninervis, respectively, which were about 3-Jan to 2-Jan the average leaf concentrations. Epidermal Cl- concentration of Halophila stipulacea was estimated to be 0.08 M, a value only about 4-Janof the overall leaf concentration.Phosphoenolpyruvate carboxylase (PEPcase) extracted from leaves of these seagrasses showed increased activity at 0.05-0.3 M NaCl in vitro. Ribulose-l, 5-bisphosphate carboxylase (RuBPcase) activity, on the other hand, was inhibited by NaCl at all tested concentrations. At epidermal NaCl concentrations, PEPcase activity was thus stimulated while RuBPcase was inhibited. The reduced RuBPcase activity at such concentrations compared to salt-free conditions was still sufficient to account for observed photosynthetic rates.We conclude that these seagrasses have adapted to a saline environment both by maintaining relatively low ion concentrations in the epidermis where photosynthesis occurs and by having carbon-fixing enzymes capable of functioning in the presence of salt.