Two high-precision continuous temperature gradient logs have been produced in a 600-m partially cased, water-filled borehole for which a nearly complete core is available. The agreement between these gradient logs, obtained using a logging speed of 8 m/minute, is good; there is virtually no dc offset (less than 0.03 degrees C/km), and the mean absolute error is approximately 0.5 degrees C/km.A comparison of these profiles with detailed geologic logs of the core shows excellent precision and resolution. In situations where the thermal resistivity contrast is 50-100 percent, isolated strata as thin as 0.5 m are clearly indicated on the temperature gradient log. More subtle lithologic changes extending over larger depth intervals are also detectable. In several instances gradient contrasts on the order of 1 degrees C/km extending over several meters have been clearly resolved. Comparison of the gradient logs with a thermal resistivity profile from laboratory measurements on the core material indicates that the steel well casing has no observable effect on the temperature gradient profile.