The fatty acid and sterol compositions of the marine tunicate Salpa thompsoni from the Antarctic Ocean are reported. Of the 21 fatty acids identified, the acids 16 : 0, 20 : 5ω3, and 22 : 6ω3 were present at high concentrations. The salpa also contained various C₂₆, C₂₇, C₂₈, and C₂₉ sterols with three Δ⁵-sterols, i.e., cholesterol (6), (22E)-(24ξ)-24-methylcholesta-5, 22-dien-3β-ol (9), and 24-methylene-cholest-5-en-3β-ol (11) as the main constituents. The presence of eight related pairs of Δ⁵-sterols and 5α-stanols with identical side chains was characteristic. Desmosterol (8) and sitostanol (or C-24 epimer) (17) were found for the first time in salpas. Furthermore, the hemolytic active components of methanol extract of the salpa were examined. The total fatty acids fraction was strongly hemolytic, and detailed examination of each component acid showed the polyunsaturated acids 22 : 6ω3 and 20 : 5ω3 to be most potent.