The screening of mutants resistant to the oxidized analogues of methionine (methionine sulphoxide and ethionine sulphoxide) allowed the characterisation of a yeast mutant strain lacking the high affinity methionine permease and defining a new locus that was called MUP1. The study of MUP1 mutants showed that methionine is transported into yeast cells by three different permeases, a high affinity and two low affinity permeases. The MUP1 gene was cloned and was shown to encode an integral membrane protein with 13 putative membrane-spanning regions. Database comparisons revealed that the yeast genome contains an ORF whose product is highly similar to the MUP1 protein. This protein is shown here to encode very low affinity methionine permease and the corresponding gene was thus called MUP3. It has previously been suggested that the amino acid permeases from yeast all belong to a single family of highly similar proteins. The two methionine permeases encoded by genes MUP1 and MUP3 are only distantly related to this family and thus define a new family of amino acid transporters.