Ion-exchange chromatography was used to examine the occurrence of glutamine synthetase (GS) isoforms in leaf tissue of 18 species of coastal plants. Two groups of species were recognizable on the basis of their complement of GS isoforms. One group comprised those species which exhibited two forms of GS, the properties of which were consistent with their being cytoplasmic and chloroplastic isoforms. In the second group were species which had only a single isoform, and exhibited properties similar to those of the chloroplastic isoform. Immunoprecipitation studies with antibody raised to the cytoplasmic isoenzyme of barley were carried out to identify the isoform present in species having only one peak of activity on ion-exchange chromatography. No relationship was evident between either total GS activity or the occurrence of the cytoplasmic isoform and the photo respiratory capacity of different species.