The nucleoli of eukaryotic cells are the sites of ribosomal RNA transcription and processing and of ribosomal subunit assembly. They contain multiple small nucleolar RNAs (snoRNAs), several of which are essential for rRNA maturation. The U3, U8 and U13 snoRNA genes are transcribed independently, whereas U14-U24, as well as E3, are located within introns of protein-coding genes, most of whose functions are linked to translation. These snoRNAs are co-transcribed with their host pre-mRNAs and released by processing from excised introns. Here we show that, in addition to U22, seven novel fibrillarin-associated snoRNAs, named U25-U31, are encoded within different introns of the unusually compact mammalian U22 host gene (UHG). All seven RNAs exhibit extensive (12-15 nucleotides) complementarity to different segments of the mature rRNAs, followed by a C/AUGA ('U-turn') sequence. The spliced UHG RNA, although it is associated with polysomes, has little potential for protein coding, is short-lived, and is poorly conserved between human and mouse. Thus, the introns rather than the exons specify the functional products of UHG.