Anacystis nidulans will take up and accumulate chloride ions. When the external concentration was 0.2 mM Cl(-) the level in the cells was 2.8 mM Cl(-) and under these conditions the flux across the cell surface was in the region of 10(-13)equiv Cl(-)·sec(-1)·cm(-2). It is suggested that this Cl(-) influx is active and operates against an electrochemical potential gradient estimated to be 117 mV or 2.68 kcal/mole. The uptake of (36)Cl was inhibited by low temperatures and there was a net loss of Cl(-) from the cells with the level decreasing towards the equilibrium value as estimated from K(+) distribution. Although the active influx of Cl(-) was often stimulated by light this was not always the case. Dark storage treatment and regulation of the chlorophyll a/phycocyanin ratios as well as total pigment content of the cells did not clarify the conditions which brought about light stimulation. Moreover, the metabolic inhibitors CCCP and CMU and also the use of anaerobic conditions did not clearly indicate the relationship between the influx mechanism and light-dark metabolism and no firm conclusions could be made about the nature of the energy source. The variation in the degree of light stimulation probably reflects the fact that in this procaryotic organism the photosynthetic and respiratory units are located on the same membrane systems and are in very close proximity to the probable site of the Cl(-) pump, the plasmalemma.