The major interest of thermal prospecting lies in the ability of an air-borne method to study large surfaces quickly. Measurements are done with a scanner radiometer borne by a plane; a wavelength band of 10.5-12.5/98m is used. Earlier experimental and theorectical studies are significant in the current limitations of this method particularly in regard to geometrical resolution, and in defining the most advisable time for measurement. The thermal survey of Lion en Beauce township was done in parallel with the study conducted by the Direction Régionale de Antiquités Historiques, referring to documentary information, interviews, and field evidence which surfaced after ploughing. Interesting data were revealed by the thermal prospecting: features of several archaeological sites can be specified; the ancient field boundaries are evident, and comparison with the 1808 land register shows which of them are older than the nineteenth century.