In the course of colonization, a reintroduced population undergoes a process of spatial expansion, which includes the development of population activity centers. A heterogeneous landscape can lead to the establishment of small partially separated sub-populations, which may suffer from loss of genetic diversity and consequently a reduction in reproductive success (RS). The wild ass was reintroduced into the Negev (1982-93). The population expanded to the Negev Heights, which is currently the main activity center (MAC). Previous study found a relation between RS and annual precipitation in Makhtesh Ramon. The goals of this research were to evaluate the genetic diversity of the Wild Ass in the MAC and to relate the RS found there to the amount of annual precipitation. RS was defined from observations and fecal samples collected in the entire distribution range were analyzed. RS in the MAC was higher than expected for Makhtesh Ramon following a drought (<40mm) during gestation (RS=0.55 vs. 0.32) and following above average precipitation years (RS=0.61 vs. 0.53, respectively), suggesting that the Negev Heights is a preferred habitat and may explain their natural expansion. While in the MAC, the frequency of mtDNA haplotypes was the same for adults and foals, it was different than samples collected in the remaining range (AMOVA: ɸ=0.016, p=0.053), with one haplotype having a low frequency (0.15). These findings imply that MAC became separated, to some extent, in a relative short time-frame and there is risk of loss of genetic diversity. Hence, a long term research is essential for monitoring changes in genetic diversity and RS of the Wild Ass throughout time.