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In extra dimension theories if the gauge bosons of the standard model propagate in the bulk of the extra dimensions then they will have Kaluza-Klein excitations that can couple to the standard model fermions. In this paper we study the effects of the first excited Kaluza-Klein mode of the W on single top production at the Tevatron. We find that the cross section for the single top production can be significantly reduced if the mass of the first Kaluza-Klein excited W ∼ 1 TeV. Hence, a measurement of the single top production cross section smaller than the standard model prediction would not necessarily imply V tb < 1 or evidence of extra generation(s) of fermions mixed with the third generation.

We consider a general effective Lagrangian for couplings of a Higgs boson to the top-quark to dimension-six operators including CP violation effects. Constraints on some of them are derived from the Z → bb data. We study the process e + e − → ttH to probe the nonstandard couplings. We find that at a linear collider with a c. m. energy √ s ∼ 0.5 − 1.5 TeV and a high luminosity of 10 − 1000 fb −1 , these non-standard couplings may be sensitively probed.

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