  We search for the decays B0->rho0rho0, B0->rho0f0, and B0->f0f0 in a sample
of about 348 million Upsilon(4S)->BBbar decays collected with the BABAR
detector at the PEP-II asymmetric-energy e+e- collider at SLAC. We find
evidence for B0->rho0rho0 with 3.0 sigma significance and measure the branching
fraction B = (1.16 +0.37-0.36 +-0.27) 10^{-6} and longitudinal polarization
fraction f_L = 0.86 +0.11-0.13 +-0.05. As a consequence, the uncertainty on the
CKM unitarity angle alpha due to penguin contributions in B->rhorho decays is
estimated to be 18 degrees at 1 sigma level. We also set upper limits on the
B0->rho0f0 and B0->f0f0 decay rates. All results are preliminary.
