28 S ribosomal RNA from several vertebrate species contains some 68 to 70 methyl groups. Evidence described in this paper enables some 58 methyl groups to be located in the primary structure of 28 S ribosomal RNA from Xenopus laevis. Most of the locations are unambiguous but a few are currently tentative. In human 28 S ribosomal RNA the great majority of the same sites are methylated as in Xenopus, but there are a few differences between the respective methyl group distributions. The main features of the methyl group distribution are as follows. (1) All of the identified methyl groups are in conserved core regions of 28 S ribosomal RNA. (2) Methylation is much more heavily concentrated in the 3' region of the molecule than in the 5' region (in contrast to 18 S ribosomal RNA, in which there is a major cluster of 2'-O-methyl groups in the 5' region). (3) In addition to the heavily methylated 3' region, clusters of methyl groups occur elsewhere in 28 S ribosomal RNA in the vicinity of domain boundaries. For domains 3 to 6, the two ends of each domain are united in a helix and are linked to adjacent domains either directly or by short single-stranded regions. It therefore follows that the methyl groups near the boundaries of these domains come together into the same general region of the three-dimensional structure. Within this large-scale pattern of distribution, methyl groups occur in a variety of local environments, examples of which are discussed. The triply methylated sequence Am-Gm-Cm-A occurs in a short single-stranded region which links domain 3 to domain 4. Near the 3' end of domain 5 there is a cluster of 11 methyl groups including a 2'-O-methyl pseudouridine in a tract of 160 nucleotides whose sequence is totally conserved between Xenopus and man. These methyl groups are variously distributed between single-stranded regions and short or imperfect but conserved helices. A further cluster of methyl groups including the highly conserved Um-Gm-psi sequence occurs in a region of domain 6 which is implicated in peptidyl transfer. Possible relationships between methylation and other events in ribosome maturation are discussed.