The mass of the top quark is measured using a sample of t (t) over bar candidate events with one electron or muon and at least four jets in the final state, collected by CMS in pp collisions at root s = 7TeV at the LHC. A total of 5174 candidate events is selected from data corresponding to an integrated luminosity of 5.0 fb(-1). For each event the mass is reconstructed from a kinematic fit of the decay products to a t (t) over bar hypothesis. The top-quark mass is determined simultaneously with the jet energy scale (JES), constrained by the known mass of the W boson in q (q) over bar decays, to be 173.49 +/- 0.43 (stat.+JES)+/- 0.98(syst.) GeV.