A description is given of the structure of the turbulent wake behind a circular cylinder that was determined using a conditional averaging technique. The structure was found for a steady flow and also for a flow in which an in‐line velocity perturbation was superimposed on the steady component. The perturbation frequency was chosen so that the frequency of vortex shedding locked‐in to the perturbation frequency. The structures of the two wakes are compared and it is found that large changes occur with lock‐in, with the strength of the vortices increasing and their lateral separation decreasing. It is estimated that the increase in the vortex strength results from an increase in the amount of vorticity shed from the cylinder and also a small change in the amount of vorticity surviving the vortex formation process. The model, which Davies [J. Fluid Mech.75, 209 (1976)] found gave a good representation of the average wake flow, was used to provide comparisons with the experimental results. In general, the agreement between the theoretical and experimental results was very satisfactory.