  Asteroseismology is found to be a excellent tool for detecting
diffusion-induced helium gradients inside main-sequence A stars. Models have
been computed for 1.6 and 2.0 M$_{\odot}$ stars with pure helium diffusion, at
different ages, so that the helium gradient lies at different depths inside the
star. The adiabatic oscillation frequencies have been analysed and compared
with those of a model without diffusion. Clear signatures of the
diffusion-induced helium gradient are found in the so-called ``second
differences" : these frequency differences present modulations due to the
partial reflexion of the sound waves on the layer where the helium gradient
takes place. A tentative application to the roAp star HD60435, which presents
enough detected oscillation frequencies for the test to be possible, is very
encouraging. The results suggest the presence of a helium gradient inside the
star, which is consistent with the idea that the triggering of the oscillations
is due to the hydrogen $\kappa$-mechanism.
