Subnanovolt resolution longitudinal and Hall voltages are measured in an ultrapure YBa2Cu3O7−δ single crystal. The Hall anomaly and the first-order vortex-lattice melting transition are observed simultaneously. Changes in the dynamic behavior of the vortex solid and liquid are correlated with features of the Hall conductivity σxy. With the magnetic field oriented at an angle from the twin boundaries, the Hall conductivity sharply decreases toward large negative values at the vortex-lattice melting transition.