Cymodocea nodosa (Ucria) Aschers. and Halophila ovalis (R. Br.) Hook. fil., two seagrass species from the Mediterranean and Red Sea, respectively, were investigated for their ability to use HCO3- and CO2 as exogenous carbon substrates in photosynthesis. Two other Red Sea species, Halodule uninervis (Forsk.) Aschers. and Syringodium isoetifolium (Aschers.) Dandy, were subjected to short time 14C pulse-chase experiments in order to classify them as C3 or C4 plants. Experiments show that Cymodocea nodosa and Halophila ovalis use HCO3- as their principal exogenous carbon source when photosynthesizing in natural conditions. Rates of net photosynthesis at saturating HCO3- concentrations, which also occur in natural seawater, were 0.4 and 0.05 μmoles O2 min-1 mg-1 chlorophyll, respectively. When additional CO2 was provided to this system, photosynthetic rates increased. This effect was especially marked in Cymodocea nodosa. In this species, photosynthetic rates were three times higher at saturating CO2 than they were at saturating HCO3- concentrations. This agrees with earlier results for different seagrasses including the two other species dealt with in this paper. In Halophila ovalis, photosynthetic rates were similar at saturating concentrations of both forms of carbon. The significance of these findings is discussed. The 14C pulse-chase experiments show that Halodule uninervis and Syringodium isoetifolium are C3 plants, even though they do possess features that might indicate C4 metabolism. These plants are unlike Thalassia testudinum Banks ex König which was shown earlier to be a C4 plant. The diversity of photosynthetic pathways in seagrasses is discussed.