The effect of aspartame (1‐aspartyl‐1‐phenylalanine, methyl ester) was studied using the protozoan, Tetrahymena pyriformis. Aspartame at levels up to 2500 ppm in a protease peptone —‐ yeast extract medium neither changed the growth rate nor rate of synthesis of DNA, RNA, protein, lipids or glycogen. Cells exposed to as much as 2500 ppm aspartame had a normal morphology. The dipeptide was adsorbed at a level of 0.0353 mg/105cells when (14C) aspartame was added at 2000 ppm to early log cultures and allowed to incubate f