Recent blue-native gel electrophoresis studies gave evidence for the existence of dimeric and trimeric PSI complexes in green plants. We used single particle electron microscopy to investigate all the larger particles from the thylakoid membrane of pea (Pisum sativum var. Charmette). Peak fractions with monomeric, dimeric and trimeric Photosystem I were obtained after solubilization with digitonin and size-exclusion chromatography. The analysis showed that only a few percent of dimers and trimers were present. In the best resolved trimers some of the monomers were oriented upside down. Many classes were fuzzy, indicating a non-specific or flexible orientation. From these results we conclude that the green plant PSI is monomeric within the green plant membrane.