  We calculate the diffusion coefficients of charged cosmic rays (CR)
propagating in regular and turbulent magnetic fields. If the magnetic field is
dominated by an isotropic turbulent component, we find that CRs reside too long
in the Galactic disc. As a result, CRs overproduce secondary nuclei like boron
for any reasonable values of the strength and the coherence length of an
isotropic turbulent field. We conclude therefore that the propagation of
Galactic CRs has to be strongly anisotropic because of a sufficiently strong
regular field and/or of an anisotropy in the turbulent field. As a consequence,
the number of sources contributing to the local CR flux is reduced by a factor
${\cal O}(100)$ compared to the case of isotropic CR diffusion.
