We describe a time-dependent three-dimensional magnetohydrodynamic (MHD) simulation of plasma flow past Jupiter's satellite Io. The simulation starts with a uniform, magnetized flow interacting with a finite conducting sphere (representing Io), and the integration is carried forward in time until a quasi-steady state develops. Previously we have reported on the presence of standing slow mode perturbations in the flow, attached to Io. In this paper we present details of the simulation model and describe the overall features of the interaction. Field-aligned currents are found in regions extending downstream from the obstacle at an angle to the background field; these currents form the Alfv¿n wings, an expected result. The plasma flow diverts both around Io and the Alfv¿n wings. An ''Alfv¿n ellipse,'' the intersection of a cylindrical Alfv¿n wing with an arbitrary plane above Io, describes the obstable that an Alfv¿n wing presents to the flow. In addition to the Alfv¿n mode, which carries the field-aligned currents, perturbations carried by the other MHD modes are also present. Standing slow mode perturbations are the most significant of these, but a fast mode signature also appears. Near Io the interaction is clearly nonlinear; all three MHD modes interact, and the local Alfv¿n and sound speeds are altered. However, estimates of the spatial location of the standing MHD mode disturbances based on the background characteristic velocities are qualitatively accurate. ¿ American Geophysical Union 1991