Suspensions of colloidal particles form a variety of ordered planar structures at an interface in response to an a.c. or d.c. electric held applied normal to the interface(1-3). This field-induced pattern formation can be useful, for example, in the processing of materials. Here we explore the origin of the ordering phenomenon. We present evidence suggesting that the long-ranged attraction between particles which causes aggregation Is mediated by electric-field-induced fluid flow. We have Imaged an axially symmetric flow field around individual particles on a uniform electrode surface. The flow is induced by distortions in the applied electric held owing to inhomogeneities in the ’double layer’ of ions and counterions at the electrode surface. The beads themselves can create these inhomogeneities, or alternatively, we can modify the electrode surfaces by lithographic patterning so as to introduce specified patterns into the aggregated structures.