The development of secondary instability on streamwise vortex structures generated in a hypersonic shock layer on a flat plate is experimentally studied for the flow with Mach number M∞ = 21 and unit Reynolds number Re1 = 6 × 105 m−1. The study is performed using the electron-beam method. The generation of weak unsteady vortices and steady streamwise vortex structures with finite-amplitude perturbations imposed onto them is studied in detail. Complex data on the characteristics of density fluctuations developed on quasi-steady and unsteady streamwise vortex structures are obtained. It is shown that the characteristics of the natural fluctuations of density developing in the shock layer on a flat plate are qualitatively similar to density fluctuations induced by weak unsteady vortex perturbations introduced into the shock layer. The possibility of existence of parametric resonance between the fundamental frequency and its harmonic and between harmonics for steady streamwise vortex structure is shown.