Fermilab-Tevatron. Measurement of the cross section of top quark pair production in PBAR P interactions at c.m. energy of 1.8 TeV. The cross section is measured in nine independent decay channels and evaluated assuming a top quark massof 172.1 GeV. The data were collected during the 1992-96 Tevatron running perio d with an integrated luminosity of approximately 125 pb-1. 80 candidate events are observed with an expected background of 38.8 +- 3.3 events.