A genetically engineered bioluminescent bacterium (lac::luxCDABE) was immobilized to develop a whole cell biosensor for the detection of toxic gaseous chemicals. The toxicity of chemicals can be evaluated through the bioluminescent reaction as it reduces in intensity when the cells experience toxic or lethal conditions. This whole cell biosensor was fabricated, using an immobilization technique utilizing solid agar medium, for the measurement of toxicity through direct contact of the cells with the gas. To enhance the sensitivity of the biosenor, glass beads were used and the thickness of the agar layer was reduced. The bioluminescent response was measured using a fiber optic probe connected between the biosensor kit and a luminometer. As sample gaseous toxic chemicals, BTEX (Benzene, Toluene, Ethylbenzene, and Xylene) gases were selected and their vapors were produced by a gas generation system. The concentrations of the gaseous chemicals injected into the chamber were controlled by the time of exposure and were measured using a portable gas chromatograph (Allstech., USA). Additions of glass beads facilitated gas diffusion through the solid medium, making the biosensor more sensitive. In addition, a thinner matrix layer was more advantageous for the detection of gas toxicity.