Restriction map and organization of rDNA was inferred from analysis of dihaploid progenies of two tetraploid genotypes of cultivated potato. Each tetraploid genotype was characterized by a specific distribution of different types of rDNA repetition units on their four homologous chromosomes II. The genotypes were heterozygous and differed by the kind of units carried by each chromosome II. Models for the generation of the observed organization are discussed and supported by first cloning studies.