The DNA of the recombinant phage lambda gtWES Mr974 (GRUMMT et al., 1979) which contains the 18S region and adjacent spacer sequences of the ribosomal genes from mouse has been digested with the restriction endonuclease SalI. Fragments corresponding to the non-transcribed spacer (A and D) and the external transcribed spacer (B) have been prepared and their nucleotide composition and sequence organization has been determined. The data indicate that the part of the non-transcribed spacer contained in Mr974 consists of at least two structural domains of distinct sequence characteristics. Fragment A contains 49% G + C and exhibits a high sequence complexity. Fragment D, the spacer fragment flanking the coding region, is very rich in G + C and is obviously composed of an internally repetitive sequence which is cut by several restriction enzymes into a similar set of repetitive fragments. Most of the fragments have sizes that are multiples of 60 and 80 or 140 base pairs, respectively, suggesting an alternating 60/80bp arrangement. This regular sequence in fragment D accounts both for the observed instability and length heterogeneity of the rDNA insert in several clones and probably for the heterogeneity in the structure of the ribosomal repeats in the genomic DNA.