We present the results of a series of balloon flights made at two latitudes over Aire sur l'Adour, France (4.5 GV), and Sao Jos¿ Campos, Brazil (12.5 GV). We used two types of omnidirectionnal detectors: a 4.5¿5.1 cm NaI crystal and a 23 cm3 Ge-Li high-energy resolution detector. The two were surrounded by a 0.8 cm thick plastic anticoincidence. These measurements allowed us to determine precisely the characteristics of the 0.511-MeV line versus altitude and latitude. We have also determined from the analysis of the growth curves at the two latitudes an extraterrestrial component value of (4.3¿1.5) ¿10-2 photons/cm2 s and an upper limit of 4¿10-3 photons/cm2 s for the flux from the galactic center region.