The mixing layer was studied at a Reynolds number (U1&dgr;*/&ngr;) of 270 and at six velocity ratios ranging from 1.0 to 0.46. Velocity measurements were carried out in the developing region of the mixing layer. Results show that naturally developing velocity fluctuations reach the maximum at a certain stream location in the developing region, then decay, and that the streamwise velocity fluctuation increases again in the further downstream region where the mean flow approaches the hyperbolic‐tangent shape. There is an indication that the fluctuating motion initiated in the wake region with its characteristic scale can pick up additional energy in the downstream hyperbolic‐tangent mean‐flow region, although stability theory predicts a most unstable length scale that is substantially larger.