The most fundamental aspects of parasitism are those concerned with nutrition. Parasitism is most frequently defined in nutritional terms. The penetration of materials of potential nutritional importance from the environment into the tissues of a parasite, therefore, constitutes a class of phenomena of first importance in parasitism. The general problems involved in differentiating simple diffusion, facilitated diffusion or transport, and active transport through living membranes are complex, and have been reviewed in recent years by Davson and Danielli (1952), Danielli (1954), Rosenberg (1954), Patlack (1956), Christensen (1958), Scheer (1958), and others. There are no critical studies of facilitated diffusion or active transport of organic substances into animal parasites. The studies of Daugherty (1957a, b) and of Daugherty and Foster (1958), purporting to demonstrate active transport of certain amino acids into Hymenolepis and Raillietina, are completely inadequate in this respect. No kinetic data were presented by these authors, and the stereospecificity observed can be explained by the assumption of weak sites of binding within the animal. Further, the effects of temperature on the amount of substance found in the worms after a fixed period of time are quite unreliable in determining how the material entered. With nonliving membranes, Qio values for diffusion may range from slightly above 1.0 to more than 40 (Davson and Danielli, 1952). On the other hand, Trim and Alexander (1949) carried out an excellent study of the kinetics of penetration of various anthelminthics through the cuticle of Ascaris. These drugs apparently enter by diffusion through a lipoid membrane. Tapeworms seem to represent unusually good material for studies of permeation. Since they lack a digestive tract, substances must enter the animal through the external surface. Tapeworms are of sufficient size so that they may be removed from test solutions after relatively short periods of time and utilized in studies of kinetics. The experiments to be reported below were carried out with the tapeworm Calliobothrium verticillatum (Tetraphyllidea) from the spiral valve of the smooth hound (Mustelus canis). The work is necessarily incomplete because of the simultaneous exhaustion of the supplies of fish, isotopically-labeled compounds, and time. Appreciation is expressed to Mr. Robert Lehy and the gentlemen of the Supply Department of the Marine Biological Laboratory, Woods Hole, Massachusetts, for their aid in obtaining infected elasmobranchs.