Cytogenetic studies were made with particular emphasis on the sex-determining mechanism in Rumex acetosella (6 x = 42) and its hybrids (F 1, F 2, BC 1 and BC 2) with R. hastatulus (synthetic 4 x = 16 = 4 A +4 X = ♀ and 4 x = 18 = 4 A + 2 (X Y 1 Y 2) = ♂). Rumex acetosella was almost strictly dioecious with 50∶50 male and female. Breeding tests revealed that the males were heterogametic. The longest chromosomes (S), usually two, are the sex chromosomes of this hexaploid species. The S chromosomes are homomorphic in both male and female. The sex chromosome: autosome ratios, and the strong epistatic male effect of the S (M) chromosome in the polyploid dioecious species and in the hybrids, are evidence of an X/Y Melandrium type sex-determining mechanism controlled by a single pair of homomorphic sex chromosomes. Thus, the sex chromosome formula of the males was S (F) S (M) and that of females was S (F) S (F). The present approach is a new method for resolving the sex-determining mechanism in a dioecious species.