The formation of vortex structures in a screen cylinder wake was investigated in a wind tunnel at a Reynolds number of 7000. The screen cylinder was made of a stainless steel wire mesh with an open area ratio of 67%. The results showed that the screen cylinder wake could be classified into two distinct regions. The first region was characterised by the development of the shear layer vortices which resulted from Kelvin-Helmholtz instability. At about x/d = 2 0 (where d is the diameter of the cylinder) the shear layer vortices started to interact with each other across the centreline, and evolved downstream to form the alternately arranged 'large-scale' coherent structures. These structures were most pronounced at x/d = 40. The vortex formation region was therefore extended significantly downstream compared with that of the solid cylinder wake. The second region involved a gradual decay of the fully-formed large-scale structures, evidenced by the weak vorticity exchange across the wake centreline.