  We present a measurement of the branching fraction for the decay
B^0->a1+(1260)pi- with a1+->pi+pi+pi- using a data sample containing 535x10^6
BBbar pairs collected with the Belle detector at the KEKB asymmetric-energy
e+e- collider operating at the Upsilon(4S) resonance. We measure the branching
fraction Br(B^0->a1+pi-)Br(a1+->pi+pi+pi-) = (14.9 +- 1.6 +- 2.3)x10^-6, where
the first and second errors are statistical and systematic, respectively.
