  We present a combination of measurements of the top quark mass by the D0
experiment in the lepton+jets and dilepton channels. We use all the data
collected in Run I (1992--1996) at $\sqrt s=1.8$ TeV and Run II (2001--2011) at
$\sqrt s=1.96$ TeV of the Tevatron $p \bar{p}$ collider, corresponding to
integrated luminosities of 0.1 fb$^{-1}$ and 9.7 fb$^{-1}$, respectively. The
combined result is: $m_t = 174.95 \pm0.40\,{\rm(stat)} \pm
0.64\,{\rm(syst)}\,{\rm{GeV}}=174.95 \pm 0.75 \, {\rm{GeV}}$.
