  We present a new measurement of the cosmic-ray positron fraction e+/(e+ + e-)
obtained from the first balloon flight of the High Energy Antimatter Telescope
(HEAT). Using a magnet spectrometer combined with a transition radiation
detector, an electromagnetic calorimeter, and time-of-flight counters we have
achieved a high degree of background rejection. Our results do not indicate a
major contribution to the positron flux from primary sources. In particular, we
see no evidence for the significant rise in the positron fraction at energies
above ~10 GeV previously reported.
