A small hydrophobic polypeptide is encoded within the genome of the alphaviruses by a set of 165 nucleotides which map between the sequences for the two virus glycoproteins. This polypeptide has been referred to as 6K and was previously found on membranes in virus-infected cells. We report here that this protein is heavily acylated with long chain fatty acids covalently attached in hydroxylamine-sensitive ester bonds and that the 6K protein can be detected in purified preparations of virions. A polyclonal rabbit serum, raised against a peptide which contained the 16 amino acids at the amino-terminus of the 6K protein, was used to identify the 6K protein in infected cells and virions. This antibody also precipitated a 4K protein which was present in Sindbis virus-infected cells but not in virions. This latter protein was shown to be an underacylated form of the 6K protein and infected cells contained about twice as much 4K as 6K. In the cell there was close to a 1:1 stoichiometry between the 4K + 6K proteins and the virus glycoproteins E1, p62, and E2, but in virions the ratio of 6K to E1 + E2 ranged from 0.08 to 0.12.