Incoherent scatter measurements performed above St. Santin (44¿3'N,2¿E) in the F2 region over the period 1971--1972 have revealed the existence of a small systematic temperature difference (10--20 K) between the electrons and the ions at night, the former being the colder. However small, this effect cannot be accounted for by statistical or systematical errors. This temperature difference reflects the tendency of the electron gas to cool faster than the ion and neutral gases and the absence of any F2 region electron heat source (such as the downward flow of energy from the protonosphere) at night.