  We report a measurement of the branching fraction e^+e^- --> B0B0bar with a
data sample of 81.7 fb^-1 collected at the Upsilon(4S) resonance with the BABAR
detector at the PEP-II asymmetric-energy e^+e^- storage ring. Using partial
reconstruction of the decay B0bar --> D^{*+} \ell^{-} \bar{\nu}_{\ell} we
obtain a preliminary result of f00 = 0.486 +- 0.010(stat.) +- 0.009(sys.). Our
result does not depend on branching fractions of the B0bar and the D^{*+} decay
chains, on the simulated reconstruction efficiency, on the ratio of the charged
and neutral B meson lifetimes, nor on assumption of isospin symmetry. This
measurement is important for normalizing many B decay branching fractions, and
contributes to our understanding of isospin violation in the Upsilon(4S)
system.
