  We present a preliminary measurement of the branching fraction of the B meson
decay B0--> a1(1260)+ pi- with a1(1260)+ --> pi+ pi+ pi- . The data were
recorded with the BaBar detector at the SLAC B factory PEP-II and correspond to
124 million B Bbar pairs produced in e+ e- annihilation through the Upsilon(4S)
resonance. We find the branching fraction Br(B0 --> a1(1260)+ pi-) = (42.6 +-
4.2 +- 4.1) 10^-6. The fitted values of the a1(1260) parameters are M_a1 = 1.19
+- 0.02 GeV/c^2 and Gamma_a1 = 312 +- 55 MeV/c^2
