  A precise measurement of the ratio of Drell-Yan yields from an 800 GeV/c
proton beam incident on liquid hydrogen and deuterium targets is reported. Over
370,000 Drell-Yan muon pairs were recorded. From these data, the ratio of
anti-down ($\bar{d}$) to anti-up ($\bar{u}$) quark distributions in the proton
sea is determined over a wide range in Bjorken-x. A strong x dependence is
observed in the ratio $\bar{d}/\bar{u}$. From this result, the integral of
$(\bar{d}-\bar{u})$ is evaluated for 0.015<x<0.35 and compared with deep
inelastic scattering results. The origin of this asymmetry is probably due to
non-perturbative effects such as those contained in pion cloud models. This
measurement has instigated new global fits since previous parameterizations of
the proton could not accommodate this large asymmetry.
