sterna hirundinacea (lesson, 1831) is a migratory seabird that breeds in the pacific coast (from peru to chile) and along the atlantic coast of south america from espírito santo (brazil) to terra del fuego (argentina). this paper describes the reproductive success of south american terns on cardos island, florianopolis, brazil in the breeding seasons of 2003, 2005 and 2006. the colony was formed in mid-may in 2003 and early april in other years, with the total number of nests ranging from 1,852 in 2006 to 2,486 in 2005. hatching success was estimated at 76.39% in 2006, 62.73% in 2003 and 41.1% in 2005, the lowest value that could be attributed to predation by hawks caracara plancus, lizards tupinambis merianae and black vulture coragyps atratus. the chicks hatched in july in 2003, and in june 2005 and 2006, and fledging success was 50.94%, 35.96 and 53.47% respectively. cardos island has been constantly used as a breeding site by south american terns, and therefore represents an important area for conservation of this species. this success could be attributed to low pressure of kelp gulls (larus dominicanus), the main predator of seabirds along the brazilian coast.