A small wire oriented orthogonally to the axis of the leading-edge vortex on a delta wing at high angle of attack generates substantial changes in the vortex structure, which is characterized using a technique of high-image-density particle image velocimetry. A wire having a diameter two orders of magnitude smaller than the diameter of the leading-edge vortex prior to the onset of vortex breakdown can substantially advance the onset of breakdown by as much as 15 vortex diameters. Depending upon the dimensionless diameter of the wire and wire location along the axis of the vortex, the onset of vortex breakdown can occur either upstream or downstream of the wire. Contours of constant velocity indicate that the rate of decrease of streamwise velocity along the centerline of the vortex is substantially enhanced, even for locations well upstream of the wire, relative to the case of vortex breakdown in the absence of a wire. Patterns of instantaneous vorticity in the presence of the wire typically exhibit a form characteristic of either a spiral- or bubble-like mode of breakdown that occurs in the absence of the wire. (C) 2001 American Institute of Physics.