Spacer-length variation in ribosomal DNA (rDNA) was surveyed in two cultivated rice species and their wild relatives. Among 243 accessions observed, 18 different spacer-length variants were detected. Length heterogeneity was found within and among species as well as within individuals. Conventional genetic analysis revealed that two spacer-length variants were located at two unlinked loci. Restriction enzyme maps showed that length heterogeneity resulted from repetition of short repeated sequences in the intergenic spacer region in the Asian cultivar and its progenitor; however, the spacer region greatly differed from those of reproductively isolated taxa with respect to the length and the sequence. Furthermore, the Asian cultivated species and its progenitor were highly polymorphic for rDNA spacer-length variation and they were differentiated in frequencies of spacer-length variants as well as varietal groups within the cultivated species. Asian cultivars tended to carry homogeneous repeats of rDNA compared with their progenitor, suggesting different forms of homogenization occurring in Asian cultivars.Key words: ribosomal DNA, intergenic spacer, polymorphism, inheritance, Oryza.