  We analyze the possibility of a spontaneous breaking of C-invariance in gauge
theories with fermions in vector-like - but otherwise generic - representations
of the gauge group. QCD, supersymmetric Yang-Mills theory, and orientifold
field theories, all belong to this class. We argue that charge conjugation is
not spontaneously broken as long as Lorentz invariance is maintained.
Uniqueness of the vacuum state in pure Yang-Mills theory (without fermions) and
convergence of the expansion in fermion loops are key ingredients. The fact
that C-invariance is conserved has an interesting application to our proof of
planar equivalence between supersymmetric Yang-Mills theory and orientifold
field theory on R4, since it allows the use of charge conjugation to connect
the large-N limit of Wilson loops in different representations.
