An improved procedure for the purification of a lipid-containing acidophilic thermophilic bacteriophage, phiNS11, is described. The procedure involves Cs2SO4 discontinuous density gradient, Cs2SO4 equilibrium density gradient and sucrose gradient centrifugations. The phage has an icosahedral shape, 75 nm in diameter, with spike-like structures radiating from the vertices. It contained about 12% by weight of lipid, 13--14% of DNA and 74--75% of protein. The phage was stable between pH 2 and 5, but rapidly inactivated above pH 6. It was highly sensitive to organic solvents and detergents. Some other basic properties such as the buoyant density and UV spectrum of the purified phage are also described.