Nucleolar dominance describes the silencing of one parent's ribosomal RNA (rRNA) genes in a genetic hybrid. In Arabidopsis thaliana, rRNA genes are clustered in two nucleolus organizer regions, NOR2 and NOR4. In F(8) recombinant inbreds (RI) of the A. thaliana ecotypes Ler and Cvi, lines that display strong nucleolar dominance inherited a specific combination of NORs, Cvi NOR4 and Ler NOR2. These lines express almost all rRNA from Cvi NOR4. The reciprocal NOR genotype, Ler NOR4/Cvi NOR2, allowed for expression of rRNA genes from both NORs. Collectively, these data reveal that neither Cvi rRNA genes nor NOR4 are always dominant. Furthermore, strong nucleolar dominance does not occur in every RI line inheriting Cvi NOR4 and Ler NOR2, indicating stochastic effects or the involvement of other genes segregating in the RI mapping population. A partial explanation is provided by an unlinked locus, identified by QTL analysis, that displays an epistatic interaction with the NORs and affects the relative expression of NOR4 vs. NOR2. Collectively, the data indicate that nucleolar dominance is a complex trait in which NORs, rather than individual rRNA genes, are the likely units of regulation.