After differential centrifugation of cell-free extracts of Chlorogloeopsis fritschii, 71% of the original glutamine synthetase (GS) activity was associated with the thylakoids, while little activity was detected in the cytoplasmic membranes. Monospecific antiserum to a purified GS inhibited 88% of the enzyme activity in solubilized thylakoid membranes. An antiserum raised against thylakoids gave 81% inhibition. However, using intact thylakoid membranes, only 7% inhibition was obtained with the GS antiserum, indicating that GS is located inside the thylakoid membranes.