In crude extracts of Chlorella kessleri, two forms of glutamine synthetase (EC 6.3.1.2) can be separated by their different surface charges. From literature data, one of them, GS 1, is considered to reside in the cytosol, and the other one, GS2, to be located in the chloroplast. Both enzyme forms are separately influenced by light and by nitrate supply: In the presence of nitrate in the culture medium (under growth conditions), GSI activity is higher but GS2 activity is lower in extracts of red light-grown cells than in those of blue light-grown cells. In extracts of white light-grown cells GS 1 and GS2 activities resemble those of extracts of blue light-grown cells. At nitrate deficiency GS2 activity is hardly detectable in extracts of algae from white light conditions while GS 1 activity is very high. Resupply of nitrate leads to an increase of GS2 only in the presence of blue light. The involvement of two different photoreceptors in the regulation of the amounts of GS 1 and GS2 of Chlorella is discussed.