  We present an analysis of two-loop mixed QCD and electroweak corrections to
the decay of the Z boson into light quarks. We find that the naive
factorization of QCD and electroweak corrections does not describe correctly
the two-loop effects. The nonfactorizable corrections shift the width of the Z
boson by approximately -0.55(3) MeV and increase the central value of the
strong coupling constant determined at LEP by 0.001.
