The direct line fluorescence of Tl, In and Ga is discussed on the basis of the possible mechanism of excitation. The importance of the energy of the metastable levels above the ground state and that of the flame temperature are stressed. Experimental results with the use of suitable filters are presented showing that, in the case of In and Ga, the thermal energy of the flame assists the fluorescence emission. At a temperature of an air-hydrogen flame given by a total consumption burner, it is shown that only for Tl a “one-way” process of excitation could be responsible for the direct line fluorescence emission. Then, two other types of atomic fluorescence, the “thermally assisted resonance fluorescence” and the “thermally assisted anti-Stokes fluorescence” are discussed.