We present a preliminary  measurement of the branching fraction of the $B$ meson decay \\
$B^0 \rightarrow a_1^+(1260) \pi^-$   with  $a_1^+(1260) \rightarrow \pi^+ \pi^+ \pi^-$.
 The data were recorded with the BaBaR  detector
at the SLAC $B$ factory PEP-II  and correspond to
$124\times 10^6$ $B \bar{B}$  pairs produced in $e^+ e^-$ annihilation through the
$\Ypsilon(4S)$ resonance.  We find the branching fraction   ${\cal B}(B^0 \rightarrow a_1^+(1260) \pi^-) = (42.6 \pm 4.2 \pm 4.1) \times 10^{-6}$.  
The fitted values of the $a_1(1260)$ parameters are $m_{a_1}=1.19 \pm 0.02$ $GeV/c^2$ and
$\Gamma_{a_1} = 312 \pm 55$ $MeV/c^2$.