Improved measurement of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mrow><mml:mover><mml:mrow><mml:mi>d</mml:mi></mml:mrow><mml:mrow><mml:mi>¯</mml:mi></mml:mrow></mml:mover></mml:mrow></mml:mrow><mml:mo>/</mml:mo><mml:mi>ū</mml:mi></mml:math>asymmetry in the nucleon sea

Measurements of the ratio of Drell-Yan yields from an $800 \mathrm{GeV}/c$ proton beam incident on liquid hydrogen and deuterium targets are reported. Approximately 360 000 Drell-Yan muon pairs remained after all cuts on the data. From these data, the ratio of down antiquark $(\overline{d})$ to up $(\ifmmode \bar{u}\else \={u}\fi{})$ antiquark distributions in the proton sea is determined over a wide range in Bjorken-$x.$ These results confirm previous measurements by E866 and extend them to lower x. From these data, $(\overline{d}\ensuremath{-}\ifmmode \bar{u}\else \={u}\fi{})$ and $\ensuremath{\int}(\overline{d}\ensuremath{-}\ifmmode \bar{u}\else \={u}\fi{})dx$ are evaluated for $0.015&lt;x&lt;0.35.$ These results are compared with parametrizations of various parton distribution functions, models and experimental results from NA51, NMC and HERMES.