 -Aminobutyric acid(GABA) is a four carbon non-protein amino acid that has several well-known physiological functions, such as a postsynaptic inhibitory neurotransmitter in the brain and induction of hypotensive and tranquilizer effects. A lactic acid bacterium was isolated from button mushroom bed, which is showing high GABA productivity by TLC or HPLC analysis. The strain was identified as Lactobacillus hilgardii by analysis of 16S rDNA gene sequence. When the maximum production of GABA by L. hilgardii was investigated with various concentration of monosodium glutamate, the yield of GABA reached to be 53.65 mM at 1% mono sodium glutamate (MSG) in flask cultivation. A Glutamate decarboxylase (GAD) enzyme, which was known to convert MSG to GABA, was purified from a cell-free extract of L. hilgardii and the molecular weights of purified GAD was estimated to 60,000 by SDS-PAGE. The optimum pH and temperature of GAD were at pH4.6 and at 37 o C, respectively. The GAD activity was increased by the addition of sulfate ions such as ammonium sulfate, sodium sulfate and magnesium sulfate, indicating that the increase of hydrophobic interaction causes the increase of GAD activity.