A sequential‐spark schlieren system with cylindrical‐lens camera was used to trace the history of transition from laminar to turbulent flow on a cylindrical model in a Mach number 1.96 air stream. Both smooth and rough models were tested at several Reynolds numbers per unit length. The results showed that transition in a supersonic stream starts with high‐frequency disturbances in the laminar boundary layer which degenerate into areas of turbulent flow. These spots erupt independently near the trailing face of the turbulent‐flow region which is always moving downstream. The addition of the newly turbulent areas of finite size to the trailing face of the turbulent‐flow region causes it to jump upstream discontinuously. Spots are traveling at a very low velocity when they first become visible and accelerate to continue downstream at a velocity of about 0.7 times free‐stream speed at their trailing face. The origin of the fresh areas of breakdown to turbulence is apparently in amplified Tollmien‐Schlichting waves. The frequency of spot production lies within the region where stability theory predicts that disturbances within the boundary layer will be amplified. It is concluded that transition mechanisms in supersonic flow are similar to those in a subsonic air stream.