Graphene-based materials are promising candidates for nanoelectronic devices(1-14) because very high carrier mobilities can be achieved without the use of sophisticated material preparation techniques(1). However, the carrier mobilities reported for single-layer and bilayer graphene are still less than those reported for graphite crystals at low temperatures, and the optimum number of graphene layers for any given application is currently unclear, because the charge transport properties of samples containing three or more graphene layers have not yet been investigated systematically(1). Here, we study charge transport through trilayer graphene as a function of carrier density, temperature, and perpendicular electric field. We find that trilayer graphene is a semimetal with a resistivity that decreases with increasing electric field, a behaviour that is markedly different from that of single-layer and bilayer graphene. We show that the phenomenon originates from an overlap between the conduction and valence bands that can be controlled by an electric field, a property that had never previously been observed in any other semimetal. We also determine the effective mass of the charge carriers, and show that it accounts for a large part of the variation in the carrier mobility as the number of layers in the sample is varied.