In earlier papers 1,2 it has been shown that morphogenetically effective blue light — compared with red light — increases protein synthesis in young fern gametophytes (= sporelings). In the present paper evidence has been presented that much of this protein which is synthesized under the influence of blue light is located in the chloroplasts. The size of the chloroplasts is correlated with the protein contents of the sporelings. Growth of the chloroplasts can be blocked by antimetabolites like 5-methyl-tryptophane, which inhibit protein synthesis. The block can be reversed by the application of tryptophane in a suitable concentration. It can be concluded from the data of this paper that only part of the protein, which originates under the influence of blue light can be regarded as specific enzyme protein in the cytoplasm. Much of the blue light dependent protein rather seems to be structural protein of the chloroplasts.