[en] We present a measurement of the t anti t production cross section in p anti p collisions at √s = 1.8 TeV and a measurement of top quark mass m_t by the D0 experiment at the Fermilab Tevatron. The measurements are based on the data from the 1992- 1996 run during which the D0 detector was exposed to the integrated luminosity of approximately 125 pb^-1. We observe 39 t anti t candidate events in the dilepton and lepton+jets decay channels with an expected background of 13.7 ± 2.2 events. We measure the top quark mass m_t using a two constraint fit to m_t in t anti t → bW⁺ anti bW^- final states with one W decaying to q anti q and the other to eν or μν. Events are binned in the fit mass versus a measure of probability for events to be the signal rather than a background. Likelihood fits to the data yield m_t = 173.3 ± 5.6 (stat) ± 6.2 (syst) GeV/c². For this mass we measure the t anti t production cross section to be 5.5 ± 1.8 pb