Components of a lipid-containing phage phiNS11 were characterized. The phage had five protein components, the molecular weights of which were 59,000, 44,000, 33,000, 23,000, and 18,000. Viral lipid consisted of six components, which were also found in the host bacterial lipid. The relative amounts of these viral lipid components were very similar to those of the bacterial lipid. The phage contained omega-cyclohexyl fatty acids characteristic of Bacillus acidocaldarius as the main fatty acids. The phage nucleic acid was a linear double-stranded DNA, the molecular weight of which was 9.3--9.4 X 10(6) daltons. The guanine plus cytosine content of the DNA was determined to be about 52% from chemical analysis, buoyant density (1.711 g/cm3 in CsCl) and melting temperature (90.6 degrees C in 0.15 M NaCl plus 0.015 M sodium citrate). The phage contained two kinds of polyamine; spermidine and spermine.