We report a measurement of the top quark mass using six candidate events fo
r the process <p(p)over bar>--><t(t)over bar>+X -->l(+) nu bl(-) <(nu b)ove
r bar> + X, observed in the D0 experiment at the Fermilab <p(p)over bar> co
llider. Using maximum likelihood fits to the dynamics of the decays, we mea
sure a mass for the top quark of m(t) = 168.4+/-12.3(stat) +/-3.6(syst) Gev
. We combine this result with our previous measurement in the <t(t)over bar
>-->l+ jets channel to obtain m(t) = 172.1+/-7.1 GeV as the best value of t
he mass of the top quark measured by D0. [S0556-2821(99)04913-9].