This paper shows that when very thin cylindrical shells are subjected to an external impulsive pressure, the interaction between the radial, purely extensional mode and the flexural modes is sufficient to precipitate permanent wrinkles. The theory predicts that these wrinkles occur at a wavelength which depends on the magnitude of the pressure pulse as well as on the cylinder parameters; experiments on cylindrical shells and thin strips confirm this prediction. Framing camera photographs showing the formation of wrinkles of this type are also presented.