We present the result of a search for charged Higgs boson decays of
the top quark, produced in ppbar collisions at sqrt s = 1.8 TeV.  When the
charged Higgs boson is heavy and decays to a tau lepton, which subsequently
decays hadronically, the resulting events have a unique signature:  large
missing transverse energy and the low-charged-multiplicity tau.  Data
collected in 1992 and 1993 at the Collider Detector at Fermilab, corresponding
to 18.7 +/- 0.7 pb^-1, exclude new regions of combined top quark and charged
Higgs boson mass, in extensions to the standard model with two Higgs doublets.