We give a simple theory for recent experiments of Bar-Ziv and Moses [Phys. Rev. Lett. 73, 1392 (1994)] in which tubular vesicles are excited using laser tweezers to a "peristaltic" state. Considering the hydrodynamics of a bilayer membrane under tension, we reproduce some of the qualitative behavior seen and find a value for the wavelength of the instability in terms of independently measured material parameters, in rough agreement with the experimental values.