The Masked Booby (Sula dactylatra) nests on tropical islands around the world (Nelson 1978, Pitman and Jehl 1998). In the eastern Pacific Ocean it nests on oceanic islands hundreds of kilometers from the coast, including the Rocas Alijos (Baja California), Clarión and San Benedicto (in the Revillagigedo Islands), and Clipperton (Pitman and Jehl 1998, AOU 1998). Its main feeding area is near and west of Clipperton and the Revillagigedo Islands, on the border of the Northern Equatorial Countercurrent (Pitman and Jehl 1998). There are no previous reports of this species breeding on near-shore islands in the eastern Pacific. Here we report the nesting of two pairs of Masked Boobies at Morros del Potosí (17° 31' N, 101° 29' W).