Effects of NaCl on growth rate, photosynthesis and respiration of Scenedesmus quadricauda were investigated, and the osmoregulation of the algae was also investigated. S. quadricauda grown under NaCl showed a reduced growth rate, and a probable increase in mitochondrial respiration with increasing NaCl concentrations. But the rate of photosyntesis in the algae, measured as the light-induced evolution of oxygen, was not affected by NaCl. To understand the osmoregulation, the NaCl concentration was selected about 50% yield as compared with the situation in the absence of NaCl. It was 100 mM NaCl. S. quadricauda has shown an ability for osmoregulation through the changes in internal osmoconcentration. Amino acids and reducing sugars were increased as S. quadricauda was grown under 100 mM NaCl. First of all, rapid accumulation of proline is one of the most remarkable metabolic consequence of high NaCl in S. quadricauda.