A strain of Penicillium notatum was cultured under various osmotic stresses.When the fungus was cultured under osmotic stress, total β-galactosidase activity increased significantly (about 1.6-fold) with increasing concentration of NaCl.Mycelial growth was repressed with increased medium osmolarity.To release periplasmic β-galactosidase, 72-h old mycelium was suspended on a concentrated solution of NaCl (0.2-1.6 M).The highest yield of β-galactosidase activity was obtained at 0.4 M NaCl at pH 6.0, which improved the activity of this enzyme by about 1.9-fold in comparison with the control medium without this depressor.