We have measured the B0B»0 mixing probability, d, using a sample of 965 000 BB» pairs from (4S) decays. Counting dilepton events, we find d=0.1570.0160.018-0.021+0.028. Using tagged B0 events, we find d=0.1490.0230.0190.010. The first (second) error is statistical (systematic). The third error reflects a 15% uncertainty in the assumption, made in both cases, that charged and neutral B pairs contribute equally to dilepton events. We also obtain a limit on the CP impurity in the Bd0 system, Re(B0)<0.045 at 90% C.L.