  We have studied the $f$-mode oscillations of differentially rotating
polytropes by making use of the linear stability analysis. We found that the
critical values of $T/|W|$ where the dynamical instability against the $m = 2$
$f$-mode oscillations sets in decrease down to $T/|W| \sim 0.20$ as the degree
of differential rotation becomes higher. Here $m$ is an azimuthal mode number
and $T$ and $W$ are the rotational energy and the gravitational potential
energy, respectively. This tendency is almost independent of the
compressibility of the polytropes. These are the {\it first exact results} of
the linear stability analysis for the occurrence of the dynamical instability
against the $m = 2$ $f$-modes.
