The gas vesicle is a hollow structure which occurs inside the cells of certain aquatic prokaryotes. It consists of a gas-filled space surrounded by a layer of protein. Most gas vesicles seem to have the same basic shape, a central cylindrical section closed with a conical cap at each end. Within the cell, gas vesicles tend to form aggregates. Under the light microscope these appear as bright refractile bodies of irregular outline and are particularly prominent when viewed with a microscope employing phase contrast optics. Gas vacuoles are found almost exclusively in the aquatic prokaryotes. The gas vesicle, rather an atypical membrane, has thus been able to provide us with some fundamental measurements on the properties of other prokaryotic cell membranes, measurements which have eluded the microbial physiologist for years. One factor which must influence the rigidity of gas vesicles is the interaction between adjacent protein molecules. The chapter describes investigations into the chemistry of the gas vesicle protein.