A detailed study is presented of resonant sum- and difference-frequency generation for the mixed-crystal system azulene in naphthalene. The S0 -> S1, S0 -> S2 and S1 -> S2 transitions of azulene provide the 3 needed dipoles for the existence of c(2), and a relatively small static elec. field was used to break the interference between the waves generated in the 2 crystal sublattices. Second-order nonlinear processes can occur sep. in each sublattice. All the theor. predicted resonances, except the DICE effect, were obsd. The generated field intensity satd. at higher fields, and in certain circumstances exhibited hysteresis as a function of the d.c. field strength. The line-narrowing capabilities of these new forms of spectroscopy were explored.