As currently known, the breeding range of Costa’s Hummingbird (Calypte costae) extends from the southwestern United States into Baja California and south to central Sonora, while the winter range extends south to the state of Nayarit (Baltosser 1989, Baltosser and Scott 1996, AOU 1998). Russell and Monson (1998) considered Costa’s Hummingbird a "poorly known species" in Mexico and reported evidence of breeding in Sonora as far south as Guásimas (27° 53' N, 110° 35' W). Cody (1983) reported observations of the species during the breeding season at Huatabampo, Sonora (26° 50' N, 109° 38' W), but did not document nesting. We report here the first evidence of breeding from southern Sonora.