Euglena cells (Euglena gracilis Z) grown heterotrophically in the dark were illuminated, and the assembly of the polypeptides of Photosystem II in the thylakoid membrane during greening of the cells was studied by sodium dodecylsulfate polyacrylamide gel electrophoresis and Western-blotting analysis with the antibodies against spinach Photosystem II polypeptides. In the dark-grown cells where the chloroplast is not developed, neither chlorophyll nor Photosystem-II-related polypeptide was detected. Upon illumination, chlorophylls and the polypeptides of Photosystem II began to be synthesized and accumulated in the thylakoid membrane. By comparing the amount of these membrane components in greening process of Euglena cells, it became evident that the extrinsic 30 kDa protein responsible for stabilization of Mn in oxygen-evolution system accumulated more slowly than the other components comprising the core complex of Photosystem II. In accordance with thiselectron transport activity from water to 2,6-dichloroin-dophenol in chloroplasts appeared later compared with that from 1,5-diphenylcarbazide to 2,6-dichloroindophenol, and the increase in the former activity was in parallel with the accumulation of the extrinsic 30 kDa protein in the thylakoid. These results suggest that accumulation and binding of the extrinsic 30 kDa protein to the core complex of Photosystem II is rate-limiting in the light-induced organization of Photosystem II and expression of the oxygen-evolution activity in Euglena. This feature is in contrast with that of higher plants and algae, where a considerable amount of extrinsic proteins of Photosystem II already exists in the dark growth condition and accumulation of the extrinsic proteins in the membrane itself is not a major determinant of the expression of oxygen evolution activity.