Abstract The uptake of α-aminoisobutyric acid in baker's yeast proceeds at the expense of metabolic energy and does not reach a steady-state level if energy and substrate are provided. The uptake shows two components, one with a K m of 5.4 mM and a V of II μmoles/g dry wt per min, the other with a K m of 0.15 mM and a V of 0.5 μmole/g dry wt per min. α-Aminoisobutyric acid does not leave the cells under any conditions, except after treatment with nystatin. The uptake is trans-inhibited by a number of different amino acids, including α-aminoisobutyric acid itself, in a non-competitive manner, the K i for α-aminoisobutyric acid vs α-aminoisobutyric acid uptake being 27 mM for the major component. A model involving two forms of carrier and strictly unidirectional fluxes is described, suggesting a feedback control by the intracellular amino acid at the key step of uptake.