Data on population size and some aspects of the breeding biology of Gentoo Penguins (Pygocelis papua) have been studied as part of a long-term monitoring program at Martillo Island in the Beagle Channel, Tierra del Fuego. The colony of Gentoo Penguin on Martillo Island is the only one reported in Argentina and has been monitored regularly since 2000. Timing and hatching success were similar to those reported at other colonies in the South Atlantic Ocean/Antarctic Peninsula region. A maximum of twelve nests were recorded during the 2006/2007 season, with an average breeding success of 1.0 chicks/nest. The Gentoo Penguin colony at Martillo Island increased by 19% in the past 13 y, while other colonies in the south Atlantic region and the Antarctic Peninsula have been decreasing or stable.