For a variety of well environments, continuous temperature logs at different speeds, taken with appropriate equipment and fast time-response probes, can yield temperature data often reproducible to several hundredths of a degree centigrade. Larger differences in reproducibility (several tenths of a degree centigrade) probably result from changes in the well bore. Below the liquid level, data need not be filtered through time-response characteristics of the probe to give accurate geothermal gradients (typically + or - a few percent) that correlate with lithology. Resulting temperature-gradient logs are qualitatively correlated to other logs, such as induction-conductivity, gamma-ray, seismic, bulk-density, and lithologic logs. Qualitative correlation with the induction-conductivity log appears best. Temperature gradients generally increase when other logs indicate the formation is becoming shaly or clayey (less sandy, silty, or limy), and/or less competent. The practical worth of the temperature-gradient log may be its ability to sense formation properties inside casing and tubing. This characteristic of the temperature-gradient log may allow lithologic identification in shut-in wells and permit useful logging after casing has been set.