The use of aluminum-jacketed optical fibers for sensitive, high-speed thermometry is demonstrated. Sensitivity of the fiber to thermal variations is observed at frequencies up to 30 kHz. Although the presence of configurationdependent mechanical resonances between 10 and 22 kHz precludes a simple analysis of the thermal response at these frequencies, the response between 5 Hz and 4 kHz is in reasonable agreement with analytic results. Assuming a minimum-detectable phase change of 10(-6) rad, the observed sensitivity at 4 kHz corresponds to a minimum detectable temperature variation of 1 microK for a 1-cm length of fiber. This performance represents orders-of-magnitude improvement in both sensitivity and frequency response relative to those of commercially available temperature-smeasuring systems.