The effects of various factors on K+ fluxes in the acid resistant unicellular green alga Dunaliella acidophila (optimal growth and photosynthesis at pH 1.0), were investigated. This algal species is capable of accumulating and regulating its K+ content (cytosolic K+ concentration is 150 mM). Data gathered suggest that K+ homeostasis is achieved mainly due to a slow efflux (0.3-1.5 nmol mgl chi-I) because the K+ uptake rates (175-325 nmol mg1 chl-1) are not much higher than those reported for other algal species. K+ uptake and efflux exhibited variable rates in cultures subjected to light and dark, to particular ionophores, SH-inhibitors and lipophilic ions. Results of the present study indicate that K+ efflux is increased significantly by induced internal acidification as seen in acetate treated algae. Apparently, the efflux mechanism is secondarily coupled to the H+ electrochemical gradient and that the uptake mechanism exerts some type of control on efflux and vice-versa.