AbstractAllozymic variation of 13 natural populations and one ex situ population of Myricaria laxiflora were analyzed using isoelectric focusing in thin-layer polyacrylamide slab gels. Thirteen loci of five enzyme systems were examined and used for the analysis of population genetic diversity and structure. A high genetic diversity was observed in this species, with mean number of alleles per locus ofA=1.8, mean percentage of polymorphic lociP=78.7% and average expected heterozygosityH_e=0.317, which were higher than the average value of Chinese endemic plant species. An excess of heterozygous genotypes was found both in natural and ex situ populations, suggesting a slight deviation from the Hardy-Weinberg equilibrium in the populations. The total genetic diversity (H_t) is 0.3733 within the species, while the genetic diversity within and among populations (H_s vs D_st) were 0.3168 and 0.0565, respectively, indicating that 84.86% of the genetic variation occured within populations and only 15.14% among natural populations. The mean gene flow among populations (N_m) was 1.401 and genetic distances among populations ranges from 0.002 to 0.176. Cluster analysis using UPGMA on Rogers' genetic distance revealed that populations of M. laxiflora growing along riversides in the Three-Gorges Reservior area were separated into upstream and downstream groups at Baishui River, Xietan region in Hubei Province. A level of genetic diversity similar to that of natural populations was observed in the ex situ population at Wuhan Botanical Garden, so the ex situ conservation program was considered to be successful in preserving genetic diversity. Combining analysis of the genetic diversity with available information on breeding system, status of natural habitat, and evolutionary history, the causes for the endangerment of M. laxiflora are discussed. It is likely that the species gradually evolved as a relict species after the Quaternary Glaciation. Based on evaluation of the current ex situ conservation program, we suggest that similar habitats upstream and downstream of the Three-Gorges Reservoir or on nearby tributaries should be selected as ex situ conservation spots for M. laxiflora populations with high genetic diversity before their original habitats are submerged by the Three-Gorges Reservoir.