A novel taurine-containing lipid was isolated from cells of Tetrahymena pyriformis NT-1. The lipid accounted for about 1% of the total lipid of the cells. The lipid was subjected to mild alkaline and methanolic hydrochloric acid hydrolyses, and the structures of the hydrolysis products were identified by mass and nuclear magnetic resonance spectrometry, as taurine, 3,7,13-trihydroxystearic acid and non-hydroxy fatty acids. By spin-decoupling analysis in nuclear magnetic resonance spectrometry, the structure of the taurine-containing lipid was identified as 2-(3-acyloxy-7,13-dihydroxyoctadecanoyl)aminoethanesulfonic acid. The radioactivity of [1,2-¹⁴C] taurine incorporated into the lipid was detected in the taurine fragment. Also, the radioactivity of [1-¹⁴C]stearic acid incorporated was found in both the 3,7,13-trihydroxystearic acid and non-hydroxy fatty acid fragments. These results suggest that the taurine-containing lipid is biosynthesized in the cells, and that stearic acid may be a precursor of the trihydroxystearic acid.