The development of the separated shear flows about rectangular and circular cylinders is investigated in the Reynolds number range 3000 ≤R 9000 by means of flow visualization with hydrogen bubbles and velocity measurement with a hot-film anemometer. The results elucidate some interesting features of the three-dimensional flow in the nonlinear stage of the transition to turbulence. The small-scale spanwise vortices formed in the separated flow undergo strong distortion and merge into a large-scale Strouhal vortex. The interaction between spanwise vortices is closely associated with the spectral properties of velocity fluctuations. The pair of the counter-rotating streamwise vortices are detected between the large-scale spanwise vortices in the near wake of the cylinder.