Plankton was collected in Nov.-Dec. 1972, during the Cato-6 Expedition (R/VMelville). Distribution of Siphonophores and Medusae and ecological implications pointed out by these organisms are discussea. 25 species of Siphonophorae were obtained corresponding to tropical, temperate, and Antarctic Sub ant arctic categories. 5 species of Medusae appeared , showing the peculiar erratic distribution typical of these organisms. Faunistic data are presented in tables and maps. Effects of Brazil, Falkland Currents and upwelling, centers appear well defined by the distribution of the siphonophores. Cosmopolitan siphonophores, typical of temperate and warm waters inhabited a band (zone of Rio Grande rise) between the southernmost and northernmost progressions of Brazil and Falkland Currents. These populations are carried toward South-american coasts by the anticyclonic circulation of the Central South Atlantic, forming a convergence; and a divergence towards the north is responsible for development of upwelling systems.