EN
Callitriche cophocarpa Sendtn. is typically a submerged macrophyte, widespread throughout Poland. It is a monoecious plant with unisexual flowers which are produced only in the floating rosettes. We studied the allozyme variation of 9 populations of C. cophocarpa from Polish rivers in order to establish its genetic structure, mode of reproduction, clonal structure, and gene flow between populations. Genetic variation of the examined populations was low. Only 7 of the 12 loci studied were polymorphic. One of the 24 detected multilocus genotypes was widespread, found in 8 populations, and the remaining genotypes were rare. The mean percentage of polymorphic loci within populations was 19.44 and the mean number of alleles per locus was 1.231, with mean values of 0.0219 observed and 0.0421 expected heterozygosity. Fixation indices calculated for each population showed a significant deficit of heterozygotes. The study showed high differentiation between populations, with almost 50% of the total variation in C. cophocarpa located between populations. Allozyme studies showed a high contribution of clonal reproduction. The populations of C. cophocarpa are isolated from each another, and the existing physical barriers decrease the probability of gene flow between populations and cross-fertilization. Gene flow within rivers is more probable.