  We report the results of a search for a charged Higgs boson in the decays of
top quarks produced in \ppbar collisions at a center-of-mass energy of 1.96
TeV. We use a data sample corresponding to an integrated luminosity of 193/pb
collected by the upgraded Collider Detector at Fermilab. No evidence for
charged Higgs production is found, allowing 95% C.L. upper limits to be placed
on BR(t->H^+b) for different charged Higgs decay scenarios. In addition, we
present in the MSSM (mH,tan beta) plane the first exclusion regions with
radiative and Yukawa coupling corrections.
