Electric field-induced second harmonic generation was applied to probe the electric field dynamics in methyl-substituted ladder-type poly(para-phenylene) (MeLPPP). To delineate the exciton and polaron contributions to the ultrafast field dynamics, we have investigated devices with traps and without them. The traps were induced upon deliberate oxidation. The relative exciton contribution to the field dynamics is larger in oxidized films due to a lower yield of mobile charge carriers.