Abstract: Sinocalycanthus chinensis is the only representative in the genus Sinocalycanthus and an endangered species restricted in small areas of Lin′an City and Tiantai County, Zhejiang Province. Currently only two natural populations remain: the larger one is found in Lin′an City, consisting of seven subpopulations with nearly 1731 990 individuals in total; the smaller one is located at Tiantai County where 2000 individuals exist. The genetic diversity of the two natural populations and one recently introduced population at Tianmushan Natural Reserve, Lin′an City, Zhejiang Province, was assessed using allozyme markers. A population of Chimonanthus zhejiangensis from Hangzhou Botanical Gardens, Hangzhou City, Zhejiang Province, was included in this study for the purpose of comparison. The genetic diversity of S. chinensis turned out to be extremely low. Only five of 23 loci from 14 enzymes assayed were polymorphic. The polymorphism was largely due to alternative fixation of alleles on Mdh-4, Pgd-3 and Sod-1, and two mutations (Gpi-1 and Gpi-2) on one individual out of 553 in total. At species level the mean number of alleles per locus ( A ) was 1.2, the percentage of polymorphic loci ( P ) was 21.7%, and the observed heterozygosity ( H_o ) was 0.010. At population level the estimates were A =1.0～1.1, P =0～13.0%, and H_o =0～0.014. In contrast, the estimates for C. zhejiangensis were much higher ( A=1.5, P=39.1%, H_o =0.071) though there were only 16 individuals tested. Since no genetic variation was detected in the introduced population of S. chinensis at Tianmushan Natural Reserve, the introduction of the plant should not be considered as a success of ex situ conservation of the endangered species. Moreover, the range of subpopulations within Longtangshan National Natural Reserve is diminishing due to the growth of evergreen forests. So far no measures have been taken to stop this subpopulation from shrinking because of lack of knowledge of ecology and biology of the plant. This study exemplifies that we are unable to conduct in situ conservation and to practise ex situ conservation properly if we have no knowledge of the biology of the species we intend to conserve.