  Using 230 million B Bbar events recorded with the BABAR detector at the e+ e-
storage rings PEP-II, we reconstruct approximately 4100 B0 to J/psi K+ pi- and
9930 B+ to J/psi K+ decays with J/psi to mu+ mu- and e+ e-. From the measured
B-momentum distributions in the e+ e- rest frame, we determine the mass
difference m(B0) - m(B+) = (+0.33 +- 0.05 +- 0.03) MeV/c^2.
