  We compute the two-loop corrections of O(alpha_s^2) to the Yukawa couplings
in the framework of the Minimal Supersymmetric Standard Model (MSSM). The
calculation is performed using the effective Lagrangian approach under the
approximation of neglecting the Higgs boson mass with respect to the top quark,
gluino and all squark flavour masses. As an application we derive the
O(alpha_s^2) corrections to the partial decay width of the lightest Higgs boson
to a bottom quark pair. We find that the two-loop corrections are sizable for
large values of tan_beta and low CP-odd Higgs boson mass. With our calculation
of the O(alpha_s^2) corrections the remaining theoretical uncertainties reduce
below a few percent.
