We measure the top quark mass mt using tt¯ pairs produced in the D0 detector by s=1.8TeVpp¯ collisions in a 125pb−1 exposure at the Fermilab Tevatron. We make a two constraint fit to mt in tt¯→bW+b¯W− final states with one W decaying to qq¯ and the other to eν or μν. Events are binned in fit mass versus a measure of probability for events to be signal rather than background. Likelihood fits to the data yield mt=173.3±5.6(stat)±6.2(syst)GeV/c2.Received 11 March 1997DOI:https://doi.org/10.1103/PhysRevLett.79.1197©1997 American Physical Society