  We report the observation of the X(3872) in the J/psi pi+pi- channel, with
J/psi decaying to mu+mu- in p-p(bar) collisions at sqrt(s) = 1.96 TeV. Using
approximately 230 pb^-1 of data collected with the Run II D0 detector, we
observe 522 +/- 100 X(3872) candidates. The mass difference between the X(3872)
state and the J/psi is measured to be 774.9 +/- 3.1 (stat.) +/- 3.0 (syst.)
MeV/c^2. We have investigated the production and decay characteristics of the
X(3872), and find them to be similar to those of the psi(2S) state.
