The Alpha Magnetic Spectrometer in a precursor version (AMS-01), was flown
in June 1998 on a 51.6 degrees orbit and at altitudes ranging between 320 a
nd 390 km, on board of the space shuttle Discovery (flight STS-91). AMS-01
included an Aerogel Threshold Cherenkov counter (ATC) to separate (p) over
bar from e and e(+) from p, for momenta below 3.5 GeV/c. This paper present
s a description of the ATC counter and reports on its: performances during
the night STS-91. (C) 2001 Elsevier Science B.V. All rights reserved.