The neutral B meson pair produced at the Y(4S) should exhibit a nonlocal correlation of the type discussed by Einstein, Podolsky, and Rosen. We measure this correlation using the time-dependent flavor asymmetry of semileptonic B-0 decays, which we compare with predictions from quantum mechanics and two local realistic models. The data are consistent with quantum mechanics, and inconsistent with the other models. Assuming that some B pairs disentangle to produce B-0 and (B) over tilde (0) with definite flavor, we find a decoherent fraction of 0.029 +/- 0.057, consistent with no decoherence.