  Let $X$ be a smooth projective curve of genus $g \geq 2$ defined over an
algebraically closed field $k$ of characteristic $p>0$. Given a semistable
vector bundle $E$ over $X$, we show that its direct image $F\_*E$ under the
Frobenius map $F$ of $X$ is again semistable. We deduce a numerical
characterization of the stable rank-$p$ vector bundles $F\_*L$, where $L$ is a
line bundle over $X$.
