Monodisperse spherical polystyrene particles were suspended at the interface between decane and water, and then subjected to extensional flow. Their lattice structure was observed to pass from a hexagonal array at rest, through a liquidlike state as flow was first applied, and finally to a new semi-ordered, anisotropic state during steady flow. This semi-ordered state was shown to be oriented and stretched in the flow direction relative to the original hexagonal structure. The influence of interfacial concentration and extensional rate on particle dynamics is discussed.