Abstract We have characterized the organization of the genes coding for 18 S, 5·8 S and 26 S ribosomal RNAs in the nematode Caenorhabditis elegans. These ribosomal genes, present in about 55 copies per haploid genome, alternate in a repeating tandem array. The repeating unit is only 7000 base-pairs, containing a non-transcribed spacer of no more than 1000 base-pairs. Most of the repeating units have identical restriction maps, but one repeat contains a deletion of 2900 base-pairs, which eliminates all or part of the 18 S coding region. We have found no difference in the major ribosomal DNA restriction endonuclease cleavage patterns between two interbreeding strains of C. elegans, but found differences between C. elegans and the closely related Caenorhabditis briggsae.