  We investigate favourable circumstances for fast magnetic reconnection in
astrophysical plasmas based on recent results by Rogers et al. (2001). Given
that a critical magnetic field structure with antiparallel field lines exists,
our analysis demonstrates that a sufficient condition for fast reconnection is
that the ratio of the thermal pressure to the magnetic field pressure $\beta$
should be significantly larger than $ 2 m_e/m_p$ (twice the ratio of electron
mass to proton mass). Using several examples (like the different components of
the interstellar medium, the intergalactic medium, active galactic nuclei and
jets) we show that in almost all astrophysical plasmas, magnetic reconnection
proceeds fast i.e. independent of the resistivity, with a few percent of the
Alfv{\'e}n speed. Only for special cases like neutron stars and white dwarfs is
$\beta$ smaller than $2 m_e/ m_p$.
