Let $G$ a compact group of isometries of a closed riemannian manifold $(X,m)$. Sunada proved that if $\Gamma_1$ and $\Gamma_2$ are two finite almost-conjugated subgroups of $G$, then $\Gamma_1\setminus X,m)$ and $\Gamma_2\setminus X,m)$ are isospectral. We prove that if $G$ is finite, there exists an open dense set in the set of $G$-invariant metrics for which the converse of this resukt is true. If $G$ is infinite, the situations is more complicated and we obtain some partial results.