  Recently, a D-brane construction in type IIA string theory was shown to yield
the electric/magnetic duality of four dimensional N=1 supersymmetric U(N_c)
gauge theories with N_f flavours of quark. We present here an extension of that
construction which yields the electric/magnetic duality for the SO(N_c) and
USp(N_c) gauge theories with N_f quarks, by adding an orientifold plane which
is consistent with the supersymmetry. Due to the intersection of the
orientifold plane with the NS-NS fivebranes already present, new features arise
which are crucial in determining the correct final structure of the dualities.
