  The lower ($H_{c1}$) and upper ($H_{c2}$) critical fields of
Mg$_{1-x}$Al$_{x}$B$_2$ single crystals (for $x = 0$, 0.1 and $\gtrsim 0.2$)
have been deduced from specific heat and local magnetization measurements,
respectively. We show that $H_{c1}$ and $H_{c2}$ are both decreasing with
increasing doping content. The corresponding anisotropy parameter
$\Gamma_{H_{c2}}(0) = H^{ab}_{c2}(0)/H^c_{c2}(0)$ value also decreases from
$\sim 5$ in pure MgB$_2$ samples down to $\sim 1.5$ for $x \gtrsim 0.2$ whereas
$\Gamma_{H_{c1}}(0)=H^c_{c1}(0)/H^{ab}_{c1}(0)$ remains on the order of 1 in
all samples. The small and large gap values have been obtained by fitting the
temperature dependence of the zero field electronic contribution to the
specific heat to the two gap model for the three Al concentrations. Very
similar values have also been obtained by point contact spectroscopy
measurements. The evolution of those gaps with Al concentration suggests that
both band filling and interband scattering effects are present.
