  A fully reconstructed Bc --> Jpsi Pi signal is observed with the D0 detector
at the Fermilab Tevatron p-pbar collider. Using 1.3 inverse femtobarns of
integrated luminosity, the signal is extracted with a significance more than
five standard deviations above background. The measured Bc meson mass is 6300
(+-) 14 (stat) (+-) 5 (sys) MeV/cc
