Unlike most of its close relatives, Arabidopsis thaliana is capable of self-pollination. In other members of the mustard family, outcrossing is ensured by the complex self-incompatibility (S) locus,which harbors multiple diverged specificity haplotypes that effectively prevent selfing. We investigated the role of the S locus in the evolution of and transition to selfing in A. thaliana. We found that the S locus of A. thaliana harbored considerable diversity, which is an apparent remnant of polymorphism in the outcrossing ancestor. Thus, the fixation of a single inactivated S-locus allele cannot have been a key step in the transition to selfing. An analysis of the genome-wide pattern of linkage disequilibrium suggests that selfing most likely evolved roughly a million years ago or more.