The relationships among specific growth rates, intracellular inorganic phosphate (Pi) and total phosphorus concentrations and specific activities of acid phosphatase (acPase) were studied on the green unicellular alga Chlorella fusca at 1.0, 15.0 and 30.0 µM Na2HPO4. The specific growth, the intracellular Pi and total phosphorus concentration increased with increasing Na2HPO4 levels. The acid phosphatase activity was inversely correlated with the specific growth rate, the total P concentrations and the intracellular inorganic concentrations. The P-deficiency induction of acPase activity is related to a decrease in P-availability in Chlorella fusca.