  We present the measurement, performed by the BABAR Collaboration, of the weak
phase alpha from the time dependent CP asymmetries in B0--> a_1(1260)+- pi-+
decays. The model error induced by penguin contributions to the B0 -->
a_1(1260)+- pi-+ channel is estimated from an SU(3) analysis of the branching
fractions of B--> a_1(1260) K, B--> K_1(1270) pi, and B--> K_1(1400) pi decays.
