We present a measurement of the top quark mass using a sample of
ttbar decays into an electron or a muon, a neutrino, and four jets.  The data
were collected in ppbar collisions at sqrt s = 1.8 TeV with the Collider
Detector at Fermilab and correspond to an integrated luminosity of 109 pb^-1.
We measure the top quark mass to be 175.9 +/- 4.8(stat) +/- 4.9(syst) GeV/c^2.