Blood glucose is a clinically important analytes for diabetic health care. In this preliminary report we describe a protein biosensor for D-glucose based on a thermostable glucose dehydrogenase. The glucose dehydrogenase was noncovalently labeled with 8-anilino-1-naphthalene sulfonic acid (ANS). The ANS-labeled enzyme displayed an approximate 25% decrease in emission intensity upon binding glucose. This decrease can be used to measure the glucose concentration, Our results suggest that enzymes which use glucose as their substrate can be used as reversible and nonconsuming glucose sensors in the absence of required cofactors, Moreover, the possibility of using inactive apoenzymes for a reversible sensor greatly expands the range of proteins which can be used as sensors, not only for glucose, but for a wide variety of biochemically relevant analytes.