U24 and U36 are members of the box C/D-containing group of antisense snoRNAs which possess long (9-21 nucleotide) conserved stretches of sequence complementarity to 18S and 28S rRNA and act as guides for the site-specific ribose methylation of rRNA. Both U24 and two variants of U36 are encoded within introns of the human and chicken rpL7a genes. We now report that an additional U36 variant is encoded within intron 4 of the human rpL7a gene and that murine homologs of the three human U36 variants are encoded within the same adjacent introns (4, 5, and 6) of the mouse rpL7a gene. We also show that, like that of the chicken, the Fugu rubripes rpL7a gene possesses only two U36-like sequences within introns 4 and 5. Whereas the two U36 variants in chicken and Fugu possess stretches of complementarity to both 18S and 28S rRNAs, it is noted that only one mammalian variant (U36b) possesses both. Unusually, the stretch of complementarity to 18S rRNA in the mammalian U36a variants and the stretch of complementarity to 28S rRNA in the mammalian U36c variants are not present, appearing to have diverged extensively from their consensus sequence. Additionally, the mammalian U36 variants show a unique heterogeneity in their potential to form a terminal stembox structure predicted for many other box C/D-containing antisense snoRNAs. Finally, the Saccharomyces cerevisiae small nuclear RNA, snR47, is shown to be homologous to the vertebrate U36 snoRNA.