Y-Aminobutyric acid(GABA) is a four carbon non-protein amino acid that has several well-known phys-iological functions, such as a postsynaptic inhibitory neurotransmitter in the brain and induction of hypotensive and tran-quilizer effects. A lactic acid bacterium was isolated from button mushroom bed, which is showing high GABAproductivity by TLC or HPLC analysis. The strain was identified as Lactobacillus hilgardii by analysis of 16S rDNAgene sequence. When the maximum production of GABA by L. hilgardii was investigated with various concentration ofmonosodium glutamate, the yield of GABA reached to be 53.65mM at 1% mono sodium glutamate (MSG) in flask cul-tivation. A Glutamate decarboxylase (GAD) enzyme, which was known to convert MSG to GABA, was purified froma 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°C, respectively. The GAD activity was increasedby the addition of sulfate ions such as ammonium sulfate, sodium sulfate and magnesium sulfate, indicating that theincrease of hydrophobic interaction causes the increase of GAD activity.