Abstract: Sagittaria potamogetifolia Merr., a species of helophytic herb limitedly distributed in Southern China, is a member of the monocotyledonous family Alismataceae1 It features monoecism with unisexual flowers, apocarpous gynoecium, self-compatibility, entomophilous pollination, and extremely high efficiency of fruit-set. For determining the genetic diversity and population differentiation of the species, eight populations were sampled from four provinces of China : Jiangxi, Hunan, Guangxi and Fu-jian, and were detected by means of allozyme analysis. Nine enzymes encoded by eighteen loci were assessed. Genetic diversity within populations was relatively high1 The genotypeson each locus were widely variant among different populations, but no locus was polymorphicous in the total population of the species1 The proportion of polymorphic loci (P) ranged from 16.67 % to 38.89 %, the average number of alleles per locus (A) from 1.278 to 1.833, the mean expected and observed heterozygosity (He, Ho) per locus were from 010941 to 011928 and 0.1461 to 0.2127 respectively1 The level of genetic diversity in studed species were higher than average values reported in some monocotyledonous species or that in some aquatic plants1 The average values of inbreeding coefficient(F =-0.2925) indicated that clonal reproduction occurred in natural populations of the species1 The coefficients of gene differentiation (Fst) in populations showed that the proportion of gene differentiation was 42.93 % among populations and 57.03 %within populations1 The ratesof gene flow (Nm) in three total populations were 013324,indicating gene flow among populations was unable to prevent the genetic differentiation between populations resulted from genetic drift1 The ge-netic identities between populations were remarkably discrepant(I = 0.7161-0.9965),and the values between neighboring pop-ulation pairs were entirely higher than 0.9000. According to the results of cluster analysis based on Neiπs genetic distances, vary close genetic distances appeared among neighboring populations, which indicated that the genetic differentiation among popula-tions of the species were related to the spatial distances1 Although the obvious morphological variances were found in neighboring populations, ecological dissimilarity did not result in genentic variance found in the enzyme loci detected.