  We present the first statistical study of the anisotropy of the magnetic
field turbulence in the solar wind between 1 and 200Hz, i.e. from proton to
sub-electron scales. We consider 93 10-minute intervals of Cluster/STAFF
measurements. We find that the fluctuations $\delta B_{\perp}^2$ are not
gyrotropic at a given frequency $f$, a property already observed at larger
scales ($\parallel$/$\perp$ mean parallel/perpendicular to the average magnetic
${\bf B_0}$). This non-gyrotropy gives indications on the angular distribution
of the wave vectors ${\bf k}$: at $f <$ 10Hz, we find that $k_{\perp}\gg
k_{\parallel}$, mainly in the fast wind; at $f >$ 10Hz, fluctuations with a
non-negligible $k_{\parallel}$ are also present. We then consider the
anisotropy ratio $\delta B_{\parallel}^2/\delta B_{\perp}^2$, which is a
measure of the magnetic compressibility of the fluctuations. This ratio, always
smaller than 1, increases with $f.$ It reaches a value showing that the
fluctuations are more or less isotropic at electron scales, for $f \geq 50$Hz.
From 1 to 15-20Hz, there is a strong correlation between the observed
compressibility and the one expected for the kinetic Alfv\'en waves (KAW),
which only depends on the total plasma $\beta$. For $f > $ 15-20Hz, the
observed compressibility is larger than expected for KAW; and it is stronger in
the slow wind: this could be an indication of the presence of a slow-ion
acoustic mode of fluctuations, which is more compressive and is favored by the
larger values of the electron to proton temperature ratio generally observed in
the slow wind.
