This article is devoted to the study of the rotating disk flow instability. This inflectional‐type instability, called a cross‐flow instability, exemplary of the transition to turbulence in three‐dimensional boundary layers. We first present the experimental marginal stability curve of unstable waves obtained by hot‐film probe measurements and compare it with the theoretical results available in the literature. The experiment is in accordance with different theoretical determinations of the linear threshold, but we note a difference between experimental and theoretical critical wave‐number values. The unstable wave dynamics is then investigated by means of experimental dispersion curves (linking frequencies to the wave‐number vector components) determined by two‐probe measurements. The results show, in particular, the existence of traveling dispersive waves in the boundary layer of the rotating disk. Finally, we show that the emergence of nonlinear effects occurs very early in the system, far from the transition point. ©1996 American Institute of Physics.