The method relies on the principles of ultrasonic reflectometry to locate and determine temperatures within zones of a slender-rod-sensing element installed in the in-situ retort bed. The measurement technique consists of sending an ultrasonic pulse down an element containing n zones spaced along its length. The zone boundaries are defined by n 1 discontinuities within the element. The time intervals between pairs of returning echoes, caused by reflections from these discontinuities, are measured electronically. These measured time intervals are then related to the average temperature of the corresponding zones. Preliminary results are encouraging, and it appears that the ultrasonic thermometer can be a useful in situ instrument.