Gleditsia caspica is an endemic treespecies found in the endangered lowlandHyrcanian forests near the Caspian Sea insoutheastern Azerbaijan and northwestern Iran. Phylogenetic analyses show that G.caspica is a derivative of G. japonica,a widely distributed species of eastern Asia. Using allozyme markers to investigate geneticdiversity within two populations fromAzerbaijan, we discovered that a populationfrom a protected nature reserve was composedlargely of first-generation hybrids betweenG. caspica and G. triacanthos,which is native to eastern North America but isplanted as an ornamental shade tree inAzerbaijan. Hybrids exhibited higher levels ofself-fertilization and lower seed productionthan either parental species. In the secondpopulation, composed of pure G. caspica,11 of 31 loci scored were polymorphic, theaverage number of alleles per locus was 1.39,and gene diversity was 0.105. All diversityestimates were substantially lower than thosewe obtained from a small sample of G.japonica from South Korea and than estimatesfrom more extensive samples of South KoreanG. japonica and North American G.triacanthos published by other workers. Weconclude that (i) in addition to ongoingthreats from habitat conversion andfragmentation, G. caspica may also bethreatened in Azerbaijan by hybridization withG. triacanthos; and (ii) low variabilityin G. caspica populations is most likelynot a consequence of recent habitat loss andreductions in population size, but insteadreflects a long history of range contractionand endemism following isolation from G.japonica during the Pliocene or Pleistocene.