We report on a measurement of the top-quark electric charge in tt 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 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.