  The lightest Higgs scalar boson mass in supersymmetry can be raised
significantly by extra vector-like quark and lepton supermultiplets with large
Yukawa couplings but dominantly electroweak-singlet masses. I consider models
of this type that maintain perturbative gauge coupling unification. The impact
of the new particles on precision electroweak observables is found to be
moderate, with the fit to Z-pole data as good or better than that of the
Standard Model even if the new Yukawa couplings are as large as their
fixed-point values and the extra vector-like quark masses are as light as 400
GeV. I study the size of corrections to the lightest Higgs boson mass, taking
into account the fixed-point behavior of the scalar trilinear couplings. I also
discuss the decay branchings ratios of the lightest new quarks and leptons and
general features of the resulting collider signatures.
