A search for narrow resonances that decay into t¯t pairs has been performed using 130 pb−1 of data in the lepton + jets channel collected by the DØ detector in p¯p collisions at √s=1.8 TeV. There is no significant deviation observed from the standard-model predictions at a top-quark mass of 175 GeV/c2. We therefore present upper limits at the 95% confidence level on the product of the production cross section and branching fraction to t¯t for narrow resonances as a function of the resonance mass MX. These limits are used to exclude the existence of a leptophobic top-color particle with mass MX<560 GeV/c2, using a theoretical cross section for a width ΓX=0.012MX.Received 29 July 2003DOI:https://doi.org/10.1103/PhysRevLett.92.221801©2004 American Physical Society