The New Energy and Industrial Technology Development Organization (NEDO) drilled well WD-1a between 1994 and 1995 in the Kakkonda geothermal field as part of their "Deep Seated Geothermal Resources Survey" project. High-temperature measurements were carried out in WD-1a. Logging temperatures above 414 degrees C were confirmed at 3600 m and 3690 m depth after 82 h standing time. Simple Horner extrapolations based on observed temperatures up to 82 h after shut-in suggested a temperature of about 500 degrees C at 3500 m depth. Temperatures between 500 degrees C and 510 degrees C were also confirmed at 3720 m depth after 129-159 h standing time, using calibrated melting tablets. These are the highest temperatures measured in a geothermal well. These results suggest a thermal structure consisting of three layers. Layer one is a shallow permeable zone of the reservoir, at less than 1500 m depth, at 230 degrees C to 260 degrees C. The second layer is a deep zone of the reservoir, which is less permeable and has a temperature of 350 degrees C to 360 degrees C from 1500 m to about 3100 m depth. The third layer is a zone of heat conduction. The transition between the hydrothermal-convection zone and the deeper heat-conduction zone is at 3100 m depth in well WD-1a.