Aims: To isolate and characterize a mutant of Streptococcus uberis strain 0140J which fails to utilize a plasmin derived beta-casein peptide for the acquisition of methionine. Methods and Results: Random insertional mutagenesis was used to isolate a mutant strain of Strep. uberis 0140J which was unable to utilize methionine from within a casein-derived peptide. The altered gene in the mutant strain showed homology to an oligopeptide permease gene of Streptococcus pyogenes (oppF). The mutant was unable to obtain specific amino acids from defined peptides of various lengths and its growth yield in skimmed milk was between 1 and 10% that of the wild-type strain, but was restored following the inclusion of these amino acids. Conclusions: The oligopeptide permease homologue of Strep. uberis 0140J is necessary for the utilization of amino acids from within specific peptides. Efficient acquisition of essential amino acids by Strep. uberis 0140J is required for the bacterium to achieve an optimum yield in milk. Significance and Impact of the Study: Streptococcus uberis is a major agent of bovine mastitis with a corresponding high economic loss. By targeting metabolic pathways essential to the growth of Strep . uberis it may be possible to prevent the establishment of growth of the bacterium in milk. This study has identified the acquisition of essential amino acids as playing a role in the growth of Strep. uberis in milk.