  We determine the top quark mass m_t using t-tbar pairs produced in the D0
detector by \sqrt{s} = 1.8 TeV p-pbar collisions in a 125 pb^-1 exposure at the
Fermilab Tevatron. We make a two constraint fit to m_t in t-tbar -> b W^+bbar
W^- final states with one W boson decaying to q-qbar and the other to e-nu or
mu-nu. Likelihood fits to the data yield m_t(l+jets) = 173.3 +- 5.6 (stat) +-
5.5 (syst) GeV/c^2. When this result is combined with an analysis of events in
which both W bosons decay into leptons, we obtain m_t = 172.1 +- 5.2 (stat) +-
4.9 (syst) GeV/c^2. An alternate analysis, using three constraint fits to fixed
top quark masses, gives m_t(l+jets) = 176.0 +- 7.9 (stat) +- 4.8 (syst)
GeV/C^2, consistent with the above result. Studies of kinematic distributions
of the top quark candidates are also presented.
