The katydids Orchelimum nigripes and O. pulchellum (Orthoptera: Tettigoniidae) have a largely allopatric distribution in the eastern United States, but hybridization between these species has been well documented in two contact zones. I used animals from populations just outside a narrow, recently formed upstream-downstream hybrid zone along the Potomac River near Washington, D.C., in laboratory mate-choice trials to examine the possible importance of differential patterns of mate preference in determining the structure and dynamics of this hybrid zone. Offered a choice between a male of each species, O. nigripes females showed an extreme preference for conspecific mates, but O. pulchellum females showed no clear preference. This asymmetry in mate discrimination may contribute to the apparently ongoing replacement along the Potomac of O. pulchellum by O. nigripes in the wake of a moving hybrid zone.