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.