  We report a new measurement of the branching fraction of neutral $B$ meson
decays $B^0\ra a_{1}^{\pm}(1260)\pi^{\mp}$ with
\ensuremath{a_{1}^{\pm}(1260)\ra \rho^{0}\pi^{\pm}} and $\rho^0\ra pi^+\pi^-$
using $275\times 10^{6}$ BBar pairs collected by the Belle detector at the
KEKBasymmetric-energy $e^{+}e^{-}$ collider. We measure the branching fraction
$\mathcal{B}($B^0\ra a_{1}^{\pm}(1260)\pi^{\mp}=(48.6\pm 4.1(stat)\pm
3.9(syst)) \times 10^{-6}$. Using a relativistic Breit-Wigner parameterization,
we measure the mass and width of the $a_{1}(1260)$ to be $m_{a_1}=1197\pm 34
MeV/c^2$ and $\Gamma_{a_1}=305\pm 43 MeV/c^2$, respectively.
