  We present a measurement of the mass difference between t and tbar quarks
observed in lepton+jets final states of ttbar events in 1 fb-1 of data
collected with the D0 detector at the Fermilab Tevatron Collider in ppbar
collisions at sqrt(s)=1.96 TeV. The observed mass difference of 3.8+-3.7 GeV is
consistent with the equality of t and tbar masses. This is the first direct
measurement of a mass difference between a quark and its antiquark partner.
