We have constructed a structural model of amorphous gallium arsenide by quenching from the melt, via first-principles molecular-dynamics simulations. The properties of our structure agree well with the available experimental information. We find that the predominant defects in this system are not wrong bonds, but threefold-coordinated atoms. Because of a relaxation mechanism similar to that occurring on the GaAs(110) surface, these do not yield states in the gap, but yield empty Ga and filled As dangling-bond states near the band edges.