The “Central-Marginal Hypothesis” posits that centrally located core populations would have higher genetic diversity compared to populations at the margin of a species’ distribution.  To test the predictions of this hypothesis, comprehensive sampling, and genetic analysis of many populations are necessary to circumscribe a species' center of genetic diversity.  Bromus tectorum L. (cheatgrass, Poaceae) is a diploid annual grass that is broadly distributed across much of Eurasia.  As part of our long-term analysis of native populations of B. tectorum, we previously analyzed the genetic diversity of 93 populations from across Eurasia, using allozymes.  By far, populations from the Middle East display the greatest genetic diversity, 4.35 multilocus genotypes (MLGs) per population, with 11 populations from Israel displaying the most diversity (31 MLGs and 5.4 MLGs/population).  To circumscribe further the center of genetic diversity of B. tectorum in its native range, we report here the analysis of 51 populations from Cyprus (12 populations), Greece (20 populations), and Turkey (19 populations).  Among these 51 populations, 40 MLGs were detected, with populations from Greece (24 MLGs and 3.1 MLGs/population) and Turkey (26 MLGs and 3.4 MLGs/population) having the greatest diversity.  Populations bordering the Aegean Sea in Greece and Turkey display the highest diversity, compared to other populations (i.e., from eastern Turkey and Cyprus).  For instance, 10 populations from western Turkey have 24 MLGs and 5.0 MLGs/ population.  Values for western Turkey populations are similar to those previously reported for populations from Israel.  Taken together, these results indicate that the center of genetic diversity for B. tectorum in its native range occurs in the eastern Mediterranean region (including areas surrounding the Aegean Sea).  These findings likely reflect the long history of human occupation, transport, and commerce throughout this region.