Carbon uptake was recorded in the light for species of the following marine algae: Alaria, Costaria, Desmarestia, Enteromorpha, Gigartina, Nereocystis, Porphyra, and Ulva. The change in total inorganic carbon in the experimental water was determined by acid release from 1 ml samples and measured by infrared gas analysis. With the exceptions of Porphyra and Desmarestia, the algae were concluded to use HCO3− ion as a substrate for photosynthesis. The relationship of rate of photosynthesis to total inorganic carbon present in seawater was determined for Ulva fenestrata, Iridaea cordata, and Sargassum muticum.