  We report on the changes of the Sun's subsurface stratification inferred from
helioseismology data. Using SOHO/MDI (SOlar and Heliospheric
Observatory/Michelson Doppler Imager) data for the last 9 years and, more
precisely, the temporal variation of f-mode frequencies, we have computed the
variation of the radius of subsurface layers of the Sun by applying
helioseismic inversions. We have found a variability of the ``helioseismic''
radius in antiphase with the solar activity, with the strongest variations of
the stratification being just below the surface around 0.995$R_{\odot}$.
Besides, the radius of the deeper layers of the Sun, between 0.975$R_{\odot}$
and 0.99$R_{\odot}$ changes in phase with the 11-year cycle.
