Abstract We discuss the weak interactions of ultra heavy fermions at low and high energy in the SU(2) L × U(1) gauge theory. Using partial-wave unitarity at high energy we establish critical fermion mass values beyond which strong couplings occur in the theory and the perturbation expansion fails. The critical masses are ( 500 √N ) GeV for quarks and ( 1.0 √N ) TeV for leptons, where N is the number of nearly degenerate SU(2) L doublets of quarks and leptons respectively. At low energies, far below their production threshold, we show that ultra heavy fermions would induce large, observable one-loop radiative corrections. One of these corrections (obtained also by Veltman) implies, using present experimental data, an upper limit of ∼700 GeV for a heavy lepton in an SU(2) L doublet with a massless neutrino.