| Summary: | Eleven 24 hour-stations were set from April 1960 through November 1962, in a certain area off the coast of Santos, Southern Brazil. The samples were collected at these stations at several depths every four hours. Sixteen different species of Hydromedusae were found, besides the Obelia spp., Solmaris spp. and Phialidium spp. Merga tergestina was newly recorded for the South Atlantic Ocean. The following species showed a marked diurnal vertical migration: Ectopleura dumortieri, Euphysora gracilis, Turritopsis nutricula, Cytaeis tetrastyla, Obelia spp., Proboscidactyla ornata, Liriope tetraphylla and Cunina octonaria. Bougainvillia frondosa, Laodicea minuscula, Phialidium spp., Amphogona apsteini, Aglaura hemistoma and Solmundella bitentaculata were found but in an insufficient number to allow detailed analyses or did not show any definitely clear migration during the present observations; though it could be inferred on the data so far accumulated that they probably migrate. The available data are not enough to describe the behaviour of Eucodonium brownie, Merga tergestina, Eucheilota duodecimalis, Bougainvillia ramosa and Solmaris spp. The thermocline appears to act as a barrier to the migrational movement of Proboscidactyla ornata. The majority of specimens of Ectopleura dumortieri, Turritopsis nutricula and Obelia spp. Migrated between the bottom and the mid-water (in the subtropical water mass). Euphysora gracilis, Liriope tetraphylla and Cytaeis tetrastyla did not migrate up to the surface when the low salinity water occupied there. The temperature and salinity ranges were given newly to some more species.
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