Two recent experimental studies investigated the s elf-excited motion Of a flat deita wing that was free to roll about an axis parallel to its mid-span chord. In this paper these experiments are s imulated numerically. An unsteady vortex-lattice method is used to provide the aerodynamic loads, and the equation of motion is integrated by a prediction-corre ction scheme. The solution provides complete histories of the motion of the wing and,the fl owfi eld simultaneously, fully accounting for dynamic-aerodyna mic interaction. The present simulation predicts that the symmetri c configuration of the 1 eadi ng-edge vortex system becomes unstable as the anyle of attack increases. Sequential views of the computed, time-dependent shape of the leading-edge system Show how this causes a loss of roll damping at small a nyles of roll. Consequently, at sufficiently high incidence, s mall disturbances i ntroduced into the flowfield grow, causing wing rock to develop. For anyles of attack at onset and amplitudes and periods of the ensuing limit cycles, the numerical predictions and the experimental o bservations are in agreement. The simulation shows the influence of the experimental parameters and provides an explanation for the differences in the observations