We present the simultaneous Swift and Fermi observations of the bright GRB 100728A and its afterglow. The early X-ray emission is dominated by a vigorous flaring activity continuing until 1ks after the burst. In the same time interval, high-energy emission is significantly detected by the Fermi/Large Area Telescope. Marginal evidence ofGeV emission is observed up to later times. We discuss the broadband properties of this burst within both the internal and external shock scenarios, with a particular emphasis on the relation between X-ray flares, theGeV emission, and a continued long-duration central engine activity as their power source. © 2011. The American Astronomical Society. All rights reserved.