This study concerns the inhibitory effects of acid pH and nickel on growth, nutrient (NO3 and NH~') uptake, carbon fixation, O~ evolution, electron transport chain and enzyme (nitrate reductase and ATPase) activities of acid tolerant and wild-type strains of Chlorella vulgaris. Though a general reduction in all these variables was noticed with decreasing pH, the tolerant strain was found to be metabolically more active than the wild-t~cpe. A reduced cation (NH~-, Na + , K + and Ca 2+) uptake, coupled with a facilitated influx of anions (NH~-, PO~-and HCO3), suggested the development of a positive membrane potential in acid tolerant Chlorella. Nevertheless, a tremendous increase in ATPase activity at decreasing pH revealed the involvement of superactive ATPase in exporting H + ions and keeping the internal pH neutral. A difference in Na + and K + efflux of the two strains at decreasing pH suggests there is a difference in membrane permeability. The low toxicity of Ni in the acid tolerant strain may be due to the low Ni uptake brought about by a change in membrane potential as well as in permeability. Hence, the development of superactive ATPase and a change in both membrane potential and permeability not only offers protection against acidity, but also co-tolerance to metals.