Glycollate : dichlorophenolindophenol reductase, or glycollate dehydrogenase, was present in 15 green marine algae and one blue-green alga collected around Lizard Island, Australia. No enzyme activity was found in red or brown algae or zooxanthellae. Glycollate dehydrogenase was also found in the marine grasses Cymodocea and Thalassia, and this is the first report of it in higher plants. However, glycollate oxidase was present in the particulate fraction from Halophila, another marine monocotyldonous plant. On Lizard Island, the C3 plants, the C4 salt plants Distichlis and Salsola, and a crassulacean acid metabolism plant, Sesuvium all contained glycollate oxidase.