  Let $f: X \rightarrow S$ be a family of non singular projective varieties
parametrized by a complex algebraic variety $S$. Fix $s \in S$, an integer $p$,
and a class $h \in {\rm H}^{2p}(X_s,\Z)$ of Hodge type $(p,p)$. We show that
the locus, on $S$, where $h$ remains of type $(p,p)$ is algebraic. This result,
which in the geometric case would follow from the rational Hodge conjecture, is
obtained in the setting of variations of Hodge structures.
