Condensed-matter systems provide a rich setting to realize Dirac(1) and Majorana(2) fermionic excitations as well as the possibility to manipulate them for potential applications(3,4). It has recently been proposed that chiral, massless particles known as Weyl fermions can emerge in certain bulk materials(5,6) or in topological insulator multilayers(7) and give rise to unusual transport properties, such as charge pumping driven by a chiral anomaly(8). A pair of Weyl fermions protected by crystalline symmetry effectively forming a massless Dirac fermion has been predicted to appear as low-energy excitations in a number of materials termed three-dimensional Dirac semimetals(9-11). Here we report scanning tunnelling microscopy measurements at sub-kelvin temperatures and high magnetic fields on the II-V semiconductor Cd3As2. We probe this system down to atomic length scales, and show that defects mostly influence the valence band, consistent with the observation of ultrahigh-mobility carriers in the conduction band. By combining Landau level spectroscopy and quasiparticle interference, we distinguish a large spin-splitting of the conduction band in a magnetic field and its extended Dirac-like dispersion above the expected regime. A model band structure consistent with our experimental findings suggests that for a magnetic field applied along the axis of the Dirac points, Weyl fermions are the low-energy excitations in Cd3As2.