  We report on a measurement of the top-quark electric charge in ttbar events
in which one W boson originating from the top-quark pair decays into leptons
and the other into hadrons. The event sample was collected by the CDF II
detector in sqrt(s)=1.96 TeV proton-antiproton collisions and corresponds to
5.6 fb^(-1). We find the data to be consistent with the standard model and
exclude the existence of an exotic quark with -4/3 electric charge and mass of
the conventional top quark at the 99% confidence level.
