  We present a measurement of the branching fraction of the decay B0 -->
a1(1260)^{+-} pi^{-+} with a1(1260)^{+-} --> pi^{-+} pi^{+-} pi^{+-}. The data
sample consists of 218 million B B-bar pairs collected at the Y(4S) resonance
and recorded by the BaBar detector at the PEP-II asymmetric-energy e+e-
collider at SLAC. The analysis is based on unbinned multivariate maximum
likelihood fit. The measured branching fraction is: (16.6 +/- 1.9 +/- 1.5)
10^{-6}. The decay mode, observed for the first time, is seen with a
significance of 9.2 sigma (systematic uncertainties included).
