Several morphological structures have been implicated in nutrient uptake in the ciliate protozoon,Tetrahymena pyriformis: food vacuoles, various types of vesicles and the plasma membrane. It is the object of this report to discuss the roles of these organelles in food uptake. Measurements of multiplication rates under conditions where food vacuole formation could be controlled experimentally suggested that the food vacuoles (about 5 μm in diameter) were essential for rapid cell multiplication in various standard growth media. If, however, concentrations of certain specific nutrients (different for different strains ofT. pyriformis) were high, then the cells could multiply rapidly even when food vacuoles were absent. Furthermore, multiplication rates of cells supplied with particulate or dissolved egg albumin as the amino acid source, suggested that the food vacuoles took up particulate egg albumin well, but dissolved egg albumin poorly. The role in food uptake of vesicles with a diameter of less than 1 μm remains largely unknown. Our present knowledge of them is not yet sufficiently detailed to permit estimations of the rates with which they are formed or of their total number per cell. The plasma membrane has carrier-mediated uptake sites for a number of nutrients such as amino acids and nucleosides. It is likely that this type of uptake mechanism plays a quantitatively important role inT. pyriformis whenever such compounds are present in the extracellular fluid.