Gluino contribution to the three loop beta function in the minimal supersymmetric standard model

We deduce the gluino contribution to the three-loop QCD {beta} function within the minimal supersymmetric standard model (MSSM) from its standard QCD expression. The result is a first step in the computation of the full MSSM three-loop {beta} function. In addition, in the case of a light gluino it provides the strong three-loop SUSY correction to the extrapolation of the strong coupling constant from the low energy regime to the Z region and up to the squark threshold. {copyright} {ital 1997} {ital The American Physical Society}