The present invention relates to a process for the continuous manufacture of vitreous synthetic silica doped with fluorine. This process consists of decomposing a silicon compound free of hydrogen in the flame of an inductive plasma burner, thereby forming silica upon reacting with the oxygen contained in the burner feed gas. A gaseous inorganic fluorine compound free of hydrogen, is sent into the flame preferably from outside the burner. Said fluorine compound simultaneously with the silicon compound decomposes whereby fluorine is introduced into the silica, lowering its index of refraction. The doped silica is then deposited on a heat-stable support in the form of a vitreous mass. The doped synthetic silica is particularly useful for making preforms for optical transmission fibers.