A thermostable soybean peroxidase-based biosensor was formed by cross-linking and electrically ''wiring'' the enzyme through a redox-conducting hydrogel to a glassy carbon electrode. At 65 degrees C the sensitivity of the sensor decayed only at a rate of only <2%/h. The sensor maintained, at 25 degrees C, the 1 A cm(-2) M(-1) sensitivity and the 0.1-200 mu M dynamic range of the earlier ''wired'' horseradish peroxidase-based sensors.