Assuming isotropic neutron scattering in beryllium, we have calculated the fundamental decay constant λ0 and the diffusion length L both in diffusion-theory approximation and according to the transport theory. A comparison of the two results enables one to comment on the range of validity of diffusion theory in crystalline moderators. We find, for example, that under the assumption of isotropic scattering, the two theories give results differing by less than 4% for B2 ≲ 0.04 cm−2 in the case of a pulsed-neutron problem and for ∑a < 0.05 cm−1 in the case of a diffusion-length problem.