Factors leading to endemism, and the evolutionary implications of endemism, can be explored by studying closely related taxa with variously restricted distributions. Such a model is provided by Carex section Ceratocystis (Cyperaceae); Carex cryptolepis, Carex sp. nov., and C. lutea are North American endemics with broad, restricted, and highly restricted distributions, respectively. The prediction that levels of genetic diversity are a consequence of distribution size was tested within a phylogenetic context using population level genetic variation at 18 allozyme loci. In contrast to expectations, mean proportion of loci polymorphic, number of alleles per polymorphic locus, and expected heterozygosity were significantly greater in C. lutea than either C. cryptolepis or Carex sp. nov. Although the possibility of a shift in breeding system, past introgression, or progenitor-derivative relationships could explain the relatively high levels of variation observed in C. lutea, these were dismissed on the basis of allozyme and nuclear ribosomal sequence data. We conclude that C. lutea maintains levels of genetic diversity typical of caespitose carices despite its narrow endemism and that the low levels of genetic variation in C. cryptolepis and Carex sp. nov. are likely the result of population fluctuations during Pleistocene glacial-interglacial cycles.