We have examined TCR V beta expression in a collection of wild mice. Many of the mice were homozygous for a large deletion at the V beta locus, and many animals also suppressed expression of several V betas using self superantigens. Expression of V beta 8.2 was unexpectedly suppressed by a self superantigen in some wild mice, which was due to the presence in these animals of a variant V beta 8.2 gene. The amino acid changes in this gene product suggest contact sites between V beta and the superantigen. Although all V betas are expressed within each wild mouse population, individual mice have a limited and variable V beta repertoire. The independent origin of multiple V beta deletions and the presence of polymorphic self superantigens suggest that this variation may be maintained by balancing selection.