Miniaturized glucose biosensors have been prepared by electrochemical codeposition of ruthenium and glucose oxidase onto carbon fiber microelectrodes (11 mu m diameter). The ruthenium sites offer an efficient and preferential catalytic action toward the electrochemical reduction of the enzymatically liberated hydrogen peroxide. Highly selective biosensing of glucose is thus accomplished at an optimal potential range around 0.0 V, where contributions from easily oxidizable substances, such as ascorbic or uric acids or acetaminophen, are eliminated. A wide linear range, up to 3 X 10(-2) M, is achieved. Electrodeposition parameters controlling the microstructure and biosensor performance are elucidated.