An enzymatic toxic gas sensor 10 having a plurality of parallel planar surfaces and a buffered electrolyte reservoir 44. The buffered electrolyte is conveyed by means of diffusion to a substrate 28 to dissolve the substrate. The substrate diffuses to an immobilized enzyme where it is hydrolyzed if the enzyme is active. An electrochemical cell continuously monitors the hydrolyzed substrate concentration which is an indication of the enzymatic activity and presence of toxic gas. A circuit means responds to the current output of the electrochemical cell to indicate the presence or absence of a toxic gas. The shelf life of the sensor is extended by means of a separator for maintaining the enzyme dry and inactive. The enzyme in the preferred embodiment is acetylcholinesterase (AcHe).