We study in detail the issue of gauge-fixing in theories with one universal
extra dimension, i.e. theories where both bosons and fermions display
Kaluza-Klein (KK) excitations. The extra dimension is compactified using the
standard orbifold construction for a massless chiral fermion. We carry out the
gauge-fixing procedure at the level of the five-dimensional theory and
determine the tree-level propagators and interaction vertices needed for
performing perturbative calculations with the effective four-dimensional theory
resulting after the compactification. The gauge-independence of the tree-level
S-matrix involving massive KK modes is verified using specific examples. In
order to obtain massive fermionic zero modes one has to enlarge the theory by
introducing a set of mirror fermions, a construction which is carried out in
detail. Finally, the gauge-independence of the tree-level S-matrix involving
the resulting new mass-eigenstates is proved by resorting to generalized
current conservation equations.Comment: 10 pages, 5 figures, revtex and axodra
