We conducted field work during May–July 2005 in the Mexican states of Nuevo León, Tamaulipas and San Luis Potosí. We observed breeding activity of Black-capped Vireos and found active nests in northwestern Nuevo León and southwestern Tamaulipas. These findings represent the first documented nests for those states, and for the latter state, the first records of Black-capped Vireos nesting below the Tropic of Cancer. Our findings suggest that the breeding range of Black-capped Vireos should be extended approximately 520 km to the southeast of the previous known nesting area near Ocampo, Coahuila.