The physical properties of the hypothesized obscuring tori
 in the nuclei of Seyfert 2 galaxies are still not understood.
 The 6.9 micron emission line of H_2 (0-0) S(5) is believed
 to be the brightest line produced in the torus. We propose to
 observe Cygnus A with ISO SWS to detect this line with AOT SWS02.
 It would enable us to constrain the physical conditions in the
 torus in Cygnus A.
 The ratio (0-0)S(5)/(1-0)S(1) would provide us with a powerful
 diagnostic of the H_2 excitation mechanism.