Summary: There are many repetitive sequences in the genomes of higher plants. The significance of repetitive sequences and their role in evolution are not clear. I analyzed tandemly arranged repetitive sequences in Rumex acetosa to obtain clues to answer these questions. In R. acetosa, the Y chromosomes are highly heterochromatic and chromosome shape polymorphisms are observed in autosomes and Y chromosomes. I discuss the role of repetitive sequences in the evolution of sex chromosomes and in morphological polymorphism of chromosomes. The morphological changes in autosomes result from an increase or decrease in the number of tandemly arranged repetitive sequences. In the sex chromosomes, specialization of the Y chromosome is promoted by the accumulation of two tandemly arranged repetitive sequences in the process of sex chromosome evolution, The morphological changesin the Y chromosomes result from a rearrangement of the chromosome arms, and do not involve a change in the number of repetitive sequences.