Measurement of the branching fraction of Upsilon(4S)->(BB0)-B-0

We report the first measurement of the branching fraction f(00) for Upsilon(4S) -> B-0(B) over bar (0). The data sample consists of 81.7 fb(-1) collected at the Upsilon(4S) resonance with the BABAR detector at the SLAC PEP-II asymmetric-energy e(+)e(-) storage ring. Using partial reconstruction of the decay (B) over bar (0) -> D(*+)l(-)<(nu)over bar >(l) in which only the charged lepton and the soft pion from the decay D*+ -> D(0)pi(+) are reconstructed, we obtain f(00) = 0.487 +/- 0.010(stat) +/- 0.008(syst). Our result does not depend on the branching fractions of (B) over bar (0) -> D(*+)l(-)<(nu)over bar >(l) and D*+ -> D(0)pi(+) decays, on the ratio of the charged and neutral B meson lifetimes, nor on the assumption of isospin symmetry.