  We report a measurement of CP-violating asymmetries in
B0-->(rhopi)0-->pi+pi-pi0 decays using a time-dependent Dalitz plot analysis.
The results are obtained from a data sample of 375 million Y(4S) --> BBbar
decays, collected by the BaBar detector at the PEP-II asymmetric-energy B
Factory at SLAC. We measure 26 coefficients of the bilinear form-factor terms
occurring in the time-dependent decay rate of the B0 meson. We derive the
physically relevant quantities from these coefficients. In particular, we
measure a constraint on the angle alpha of the Unitarity Triangle.
