Mitochondrial, microsomal and pellicular membranes were isolated from Tetrahymena cells grown at 39°C or 15°C, and phospholipids, in turn, were separated from total lipids extracted from these membranes. The effect of growth temperature on their solid-to-fluid phase transition temperature was examined by wide-angle X-ray diffraction. The transition temperatures of phospholipids from mitochondria, microsomes and pellicles were 21, 19 and 26°C for cells grown at 39°C and −8, −3 and 6°C for cells grown at 15°C, respectively. All phospholipids were found in a completely fluid state at these growth temperatures. From a comparison between the phospholipids and total lipids from pellicles of cells grown at 39°C, a triterpenoid alcohol, tetrahymanol, caused the transition temperature to increase. The alignment of tetrahymanol in membranes was examined with pellicle's total lipid oriented in a sample holder.