The geothermal convector (GTC) is an unconventional system for heat extraction from a geothermal aquifer without fluid withdrawal. In this system a special two-phase thermosyphon is used. This device works with a downhole evaporator and with a condenser at the ground level. The system shares advantages and disadvantages with other downhole heat exchanger plants; for instance, a second well to return geothermal water to the aquifier is not required. Because of the limitations of the natural heal flow in the hot aquifer, however, the device is most suitable for small-to moderate-size applications. In this article, the fundamental elements required to determine the device performance and the thermal design criteria are reported and a GTC prototype is described. Designed for 70°C temperature of the geothermal water and 3 m length of the evaporator, this prototype was recently installed in a geothermal well on the island of Ischia. In this article the results of the first experimental tests are reported.