  The angular and temperature dependence of the upper critical field Hc2 in
MgB2 was determined from torque magnetometry measurements on single crystals.
The Hc2 anisotropy gamma_H was found to decrease with increasing temperature,
in disagreement with the anisotropic Ginzburg-Landau theory, which predicts
that the gamma_H is temperature independent. This behaviour can be explained by
the two band nature of superconductivity in MgB2. An analysis of measurements
of the reversible torque in the mixed state yields a field dependent effective
anisotropy gamma_eff, which can be at least partially explained by different
anisotropies of the penetration depth and the upper critical field. It is shown
that a peak effect in fields of about 0.85 Hc2 is a manifestation of an
order-disorder phase transition of vortex matter. The H-T phase diagram of MgB2
for H//c correlates with the intermediate strength of thermal fluctuations in
MgB2, as compared to those in high and low Tc superconductors.
