[en] The complete O(N_fα²) weak contributions to the muon lifetime, denoted as Δr⁽²⁾, are calculated in the MS-bar renormalization scheme. These come from 2-loop Feynman diagrams containing a loop formed by complete generations of massless fermions. They form an independent, gauge-invariant set of corrections that, because of the large number of light fermions in the Standard Model, is expected to make a significant contribution. In the MS-bar renormalizaton scheme with μ' -- (πe^γ)^((1)/(2)) μ = M_Z and for a Higgs mass, M_H, in the range 100-1000 GeV the contribution varies from -0.55 x 10^-4 to -1.54 x 10^-4 for each massless generation of fermions