  A precision measurement by AMS of the antiproton-to-proton flux ratio in
primary cosmic rays in the absolute rigidity range from 1 to 450 GV is
presented based on $3.49\times10^5$ antiproton events and $2.42\times10^9$
proton events. Above $\sim60$ GV the antiproton to proton flux ratio is
consistent with being rigidity independent. A decreasing behaviour is expected
for this ratio considering the traditional models for the secondary antiproton
flux.
