ABSTRACT Anacystis nidulans (Richt.) Drouet & Daily (UTEX 625), grown in batch culture with 0.5% CO2 in air, was supplied with chloride labelled with 36Cl in light and dark. Uptake in light was stimulated relative to uptake in darkness. A single transport system for Cl− with an apparent Km for Cl− of 0.14 mM was identified. Chloride in the cells reached a maximum value after 30–50 min at 25 C. At this point the internal Cl− concentration was calculated to be 60‐fold the external (0.1 mM) in light and 37‐fold in darkness. DCMU (3‐[3,4‐dichlorophenyl]–1, 1‐dime‐thylurea), at concentrations which abolished photosynthetic O2 evolution did not inhibit Cl− uptake in light. Carbonyl cyanide m‐chlorophenyl hydrazone (CCCP), at uncoupling concentrations for photosynthesis and dark respiration, strongly inhibited Cl− uptake in light and darkness. N,N'‐dicyclohexyl carbodiimide (DCCD), an energy transfer inhibitor, inhibited light Cl− uptake more slowly than photosynthesis but had no effect on dark Cl− uptake. It is concluded that Cl− uptake in A. nidulans was active in light and darkness, and that ATP was the probable energy source for transport.