  We give a simple theory for recent experiments of Bar-Ziv and Moses% Phys.
Rev. Lett. {\bf73} (1994) 1392, 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.
