Two measurements of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="italic">B</mml:mi></mml:mrow><mml:mrow><mml:mn>0</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math><i>B</i><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">¯</mml:mi></mml:mrow><mml:mrow><mml:mn>0</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>mixing

We have measured the ${\mathit{B}}^{0}$B${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}^{0}$ mixing probability, ${\mathrm{\ensuremath{\chi}}}_{\mathit{d}}$, using a sample of 965 000 BB\ifmmode\bar\else\textasciimacron\fi{} pairs from \ensuremath{\Upsilon}(4S) decays. Counting dilepton events, we find ${\mathrm{\ensuremath{\chi}}}_{\mathit{d}}$=0.157\ifmmode\pm\else\textpm\fi{}0.016\ifmmode\pm\else\textpm\fi{}0.${018}_{\mathrm{\ensuremath{-}}0.021}^{+0.028}$. Using tagged ${\mathit{B}}^{0}$ events, we find ${\mathrm{\ensuremath{\chi}}}_{\mathit{d}}$=0.149\ifmmode\pm\else\textpm\fi{}0.023\ifmmode\pm\else\textpm\fi{}0.019\ifmmode\pm\else\textpm\fi{}0.010. The first (second) error is statistical (systematic). The third error reflects a \ifmmode\pm\else\textpm\fi{}15% uncertainty in the assumption, made in both cases, that charged and neutral B pairs contribute equally to dilepton events. We also obtain a limit on the CP impurity in the ${\mathit{B}}_{\mathit{d}}^{0}$ system, \ensuremath{\Vert}Re(${\mathrm{\ensuremath{\epsilon}}}_{\mathit{B}}^{0}$)\ensuremath{\Vert}0.045 at 90% C.L.