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Larvae of Taenia crassiceps have been shown to absorb L-valine and L-methionine by a process of active transport. Uptake is maximal at about 43 °C and a pH of about neutrality, and concentration against a gradient has been demonstrated. In advanced larvae, the absorption mechanism has been shown to be specific for neutral aliphatic amino acids, with an uncharged amino group in the α-position. Competitive inhibitions of uptake have been demonstrated in the presence of other amino acids.Comparison has been made with a mechanism present in Hymenolepis diminuta for the uptake of neutral amino acids, demonstrated by Read et al. (1963), and the absorption of L-methionine by the two species has been compared in detail.The uptake of valine and methionine by the two types of larvae has been compared in detail: the presence of two types of sites for the uptake of these aminoacids has been suggested.The absorption of the L-isomers of glutamic acid, proline, serine, glycine and leucine has been demonstrated in advanced larvae. Separate mechanisms for the uptake of acidic amino acids, basic amino acids and neutral aromatic amino acids have been suggested.Thanks are due to Professor G. Chapman, in whose department this work was carried out, and to Miss E. Clarry for her willing assistance. We would also like to thank Dr G. Ayrey, of the Queen Elizabeth College Isotope Unit, for his valuable help, especially in the development of a technique for the simultaneous counting of 14C and 3H. The work was supported by a research grant from the Wellcome Trustees, to whom we are most grateful.