The behavior of two‐dimensional periodic disturbances introduced artificially into turbulent shear flows is examined in a wind tunnel using flow visualization methods. It is found that when the wavelength of the disturbances is about 20 &dgr;* in the case of the boundary layer on a flat plate and about 2.4 &dgr;* in the case of the wake behind a circular cylinder, the disturbances are strongly amplified in the downstream direction, where &dgr;* is the displacement thickness of the shear layer. The present experiments suggest that the large‐scale coherent motions in natural turbulent shear flows originate from the instability of the mean velocity distributions.