  We present a measurement of the branching fraction of neutral B meson
decaying to final states containing a K_1 meson, i.e. K_1(1270) and K_1(1400),
and a charged pion. The data, collected with the BaBar detector at the Stanford
Linear Accelerator Center, represent 454 million BBbar pairs produced in e+e-
annihilation. We measure the branching fraction BF(B0 -> K_1+ pi-) = (31.0 +-
2.7 +- 6.9) x 10^-6, where the first error quoted is statistical and the second
is systematic. In the framework of the K-matrix formalism used to describe
these decays, we also set limits on the ratio of the production constants for
the K_1(1270)+ and K_1(1400)+ mesons in B0 decays.
