This study is related the behavior of 12 species of grasses used as pasture on the tropical and semiarid conditions of Piracicaba (rainfall 1.300 mm, average anual temperature: 20°C, minimum 0ºC, maximum 35°C). Agressivity is emphazised not only as a good characteristic for permanente pasture, but too as a possibility for the forage to become a serious weed. A particular study of agressivity is presented, the economic aspect is considered, and finally local observations of the behavior of the following species are discussed: "Big Colonia" (a "Guinea grass" variety), "Evergreen" (another variety of "Guinea Grass"), "Jaraguá grass", "Para grass", "Bermuda grass", "Kikuiu grass", "Bahia grass", "Torpedo grass", "Natal grass", "Rhodes grass" and "Johnson grass", and "Molasses grass".