The seaside sparrow, Ammodramus maritimus, is known to breed only as far south as Nueces and Copano bays, Texas. Systematic point counts conducted in the spring and summer of 1999 on the Boca Chica Tract of the Lower Rio Grande Valley National Wildlife Refuge revealed 3 attempted seaside sparrow nests, seaside sparrow eggshell fragments, and at least 5 seaside sparrows. Presented here is probable evidence of this species breeding near Brownsville, Texas, over 190 km south of its known breeding range.