The three-dimensional transition of the ¿ow behind a circular cylinder with a near-wake wire disturbance has been investigated experimentally. The asymmetric placement of a wire in the near-wake region of the cylinder causes an unnatural mode of shedding to occur, namely mode C. We performed ¿ow visualization and particle image velocimetry (PIV) experiments to investigate the in¿uence of the wire on various properties of the ¿ow, such as the dynamics of the streamwise secondary vortices. Experiments were performed at the Reynolds number range of Re=165–300. From these experiments, it can be concluded that mode C structures are formed as secondary streamwise vortices around the primary von Karman vortices. The spanwise wavelength of those mode C structures is determined to be approximately two cylinder diameters. The presence of the wire also triggered the occurrence of period doubling in the wake. Each new set of mode C structures is out of phase with the previous set, i.e. doubling the shedding period. This period-doubling phenomenon is due to a feedback mechanism between the consecutively shed upper vortices.