The two-dimensional melting transition of charged polystyrene spheres in aqueous colloidal suspension has been studied by molecular dynamics simulation of a screened Coulomb system. Some central predictions of the Kosterlitz-Thouless-Halperin-Nelson-Young theory of defect-mediated melting are confirmed, such as an apparent divergence of the correlation lengths for translational and bond-orientational order at different thermodynamic state points, but there are also predictions of the theory that are violated. The defect topology is very complex, with oscillation periods of the defect density of many million time steps duration. The need for extensive sampling and, to a lesser degree, the choice of potential function continue to be the crucial issues for any attempt to generate a hexatic structure by means of computer simulation.